Prof. Vahid Sandoghdar

Higher order transient membrane protein structures

Yuxi Zhang, Hisham Mazal, Venkata Shiva Mandala, Gonzalo Perez-Mitta, Vahid Sandoghdar, Christoph A. Haselwandter, Roderick McKinnon

This study shows that five membrane proteins—three GPCRs, an ion channel, and an enzyme—form self-clusters under natural expression levels in a cardiac-derived cell line. The cluster size distributions imply that these proteins self-oligomerize reversibly through weak interactions. When the concentration of the proteins is increased through heterologous expression, the cluster size distributions approach a critical distribution at which point a phase transition occurs, yielding larger bulk phase clusters. A thermodynamic model like that explaining micellization of amphiphiles and lipid membrane formation accounts for this behavior. We propose that many membrane proteins exist as oligomers that form through weak interactions, which we call higher-order transient structures (HOTS). The key characteristics of HOTS are transience, molecular specificity, and a monotonically decreasing size distribution that may become critical at high concentrations. Because molecular specificity invokes self-recognition through protein sequence and structure, we propose that HOTS are genetically encoded supramolecular units.

Cryogenic light microscopy with Ångstrom precision deciphers structural conformations of PIEZO1

Hisham Mazal, Alexandra Schambony, Vahid Sandoghdar

Despite the impressive progress in molecular biochemistry and biophysics, many questions regarding the conformational states of large (transmembrane) protein complexes persist. In the case of the PIEZO protein, investigations by cryogenic electron microscopy (Cryo-EM) and atomic force microscopy (AFM) have established a symmetric trimer structure with three long-bladed domains in a propeller-like configuration. A transition of PIEZO protein from curved to flat conformation is hypothesized to actuate closed and open channels for the flow of ions. However, conclusive high-resolution data on the molecular organization of PIEZO in its native form are lacking. To address this shortcoming, we exploit single-particle cryogenic light microscopy (spCryo-LM) to decipher the conformational states of the mouse PIEZO1 protein (mPIEZO1) in the cell membrane. Here, we implement a high-vacuum cryogenic shuttle to transfer shock-frozen unroofed cell membranes in and out of a cryostat for super-resolution microscopy at liquid helium temperature. By localizing fluorescent labels placed at the extremities of the three blades with Ångstrom precision, we ascertain three configurations of the protein with radii of 6, 12, and 20 nm as projected onto the membrane plane. Our data suggest that in the smallest configuration, the blades form a nano-dome structure that is more strongly curved than previously observed and predicted by AlphaFold-3. In the largest conformation, we believe the structure must fully unbend in an anticlockwise manner to form a flat extended state. We attribute the 12 nm conformation, the most frequently occupied state, to an intermediate state and discuss our results in the context of the findings from other groups. Combination of spCryo-LM and Cryo-EM measurements together with in situ photothermal stimulation promises to provide quantitative insight into the interplay between structure and function of PIEZO and other biomolecular complexes in their native environments.

Supported Lipid Bilayers as Stochastic Conveyor Belt for Delivery to the Near Field of Nanoscopic Structures

Yazgan Tuna, Vahid Sandoghdar

to appear

Long-Range Three-Dimensional Tracking of Nanoparticles Using Interferometric Scattering Microscopy

Kiarash Kasaian, Mahdi Mazaheri, Vahid Sandoghdar

Tracking nanoparticle movement is highly desirable in many scientific areas, and various imaging methods have been employed to achieve this goal. Interferometric scattering (iSCAT) microscopy has been particularly successful in combining very high spatial and temporal resolution for tracking small nanoparticles in all three dimensions. However, previous works have been limited to an axial range of only a few hundred nanometers. Here, we present a robust and efficient measurement and analysis strategy for three-dimensional tracking of nanoparticles at high speed and with nanometer precision. After discussing the principle of our approach using synthetic data, we showcase the performance of the method by tracking gold nanoparticles with diameters ranging from 10 to 80 nm in water, demonstrating an axial tracking range from 4 μm for the smallest particles up to over 30 μm for the larger ones. We point out the limitations and robustness of our system across various noise levels and discuss its promise for applications in cell biology and material science, where the three-dimensional motion of nanoparticles in complex media is of interest.

iSCAT microscopy and particle tracking with tailored spatial coherence

Mahdi Mazaheri, Kiarash Kasaian, David Albrecht, Jan Renger, Tobias Utikal, Cornelia Holler, Vahid Sandoghdar

Interferometric scattering (iSCAT) microscopy has demonstrated unparalleled performance among label-free optical methods for detecting and imaging isolated nanoparticles and molecules. However, when imaging complex structures such as biological cells, the superposition of the scattering fields from different locations of the sample leads to a speckle-like background, posing a significant challenge in deciphering fine features. Here, we show that by controlling the spatial coherence of the illumination, one can eliminate the spurious speckle without sacrificing sensitivity. We demonstrate this approach by positioning a rotating diffuser coupled with an adjustable lens and an iris in the illumination path. We report on imaging at a high frame rate of 25 kHz and across a large field of view of 100µm×100µm, while maintaining diffraction-limited resolution. We showcase the advantages of these features by three-dimensional (3D) tracking over 1000 vesicles in a single COS-7 cell and by imaging the dynamics of the endoplasmic reticulum (ER) network. Our approach opens the door to the combination of label-free imaging, sensitive detection, and 3D high-speed tracking using wide-field iSCAT microscopy.

High-Resolution Cryogenic Spectroscopy of Single Molecules in Nanoprinted Crystals

Mohammad Musavinezhad, Jan Renger, Johannes Zirkelbach , Tobias Utikal, Claudio U. Hail, Thomas Basché, Dimos Poulikakos, Stephan Götzinger, Vahid Sandoghdar

We perform laser spectroscopy at liquid helium temperatures (T = 2 K) to investigate single dibenzoterrylene (DBT) molecules doped in anthracene crystals of nanoscopic height fabricated by electrohydrodynamic dripping. Using high-resolution fluorescence excitation spectroscopy, we show that zero-phonon lines of single molecules in printed nanocrystals are nearly as narrow as the Fourier-limited transitions observed for the same guest–host system in the bulk. Moreover, the spectral instabilities are comparable to or less than one line width. By recording super-resolution images of DBT molecules and varying the polarization of the excitation beam, we determine the dimensions of the printed crystals and the orientation of the crystals’ axes. Electrohydrodynamic printing of organic nano- and microcrystals is of interest for a series of applications, where controlled positioning of quantum emitters with narrow optical transitions is desirable.

Measuring Concentration of Nanoparticles in Polydisperse Mixtures Using Interferometric Nanoparticle Tracking Analysis

Anna D. Kashkanova, David Albrecht, Michelle Küppers, Martin Blessing, Vahid Sandoghdar

Quantitative measurements of nanoparticle concentration in liquid suspensions are in high demand, for example, in the medical and food industries. Conventional methods remain unsatisfactory, especially for polydisperse samples with overlapping size ranges. Recently, we introduced interferometric nanoparticle tracking analysis (iNTA) for high-precision measurement of nanoparticle size and refractive index. Here, we show that by counting the number of trajectories that cross the focal plane, iNTA can measure concentrations of subpopulations in a polydisperse mixture in a quantitative manner and without the need for a calibration sample. We evaluate our method on both monodisperse samples and mixtures of known concentrations. Furthermore, we assess the concentration of SARS-CoV-2 in supernatant samples obtained from infected cells.

Lipidic folding pathway of α-Synuclein via a toxic oligomer

Christian Griesinger, Vrinda Sant, Dirk Matthes, Hisham Mazal, Leif Antonschmidt, Franz Wieser, Kumar Tekwani Movellan, Kai Xue, Evgeny Nimerovsky, et al.

Aggregation intermediates play a pivotal role in the assembly of amyloid fibrils, which are central to the pathogenesis of eurodegenerative diseases1,2. The structures of filamentous intermediates3 and mature fibrils4 are now efficiently determined by single-particle cryo-electron microscopy. By contrast, smaller pre-fibrillar α-Synuclein (αS) oligomers, crucial for initiating amyloidogenesis, remain largely uncharacterized. We report an atomic-resolution structural characterization of a toxic pre-fibrillar aggregation intermediate (I1) on pathway to the formation of lipidic fibrils. Super-resolution microscopy reveals a tetrameric state, providing insights into the early oligomeric assembly. Time resolved nuclear magnetic resonance (NMR) measurements uncover a structural reorganization essential for the transition of I1 to mature lipidic L2 fibrils. The reorganization involves the transformation of anti-parallel β-strands during the pre-fibrillar I1 state into a β-arc characteristic of amyloid fibrils. This structural reconfiguration occurs in a conserved structural kernel shared by a vast number of αS-fibril polymorphs including extracted fibrils from Parkinson’s and Lewy Body Dementia patients. Consistent with reports of anti-parallel β-strands being a defining feature of toxic αS pre-fibrillar intermediates6, I1 impacts viability of neuroblasts and disrupts cell membranes, resulting in an increased calcium influx. Our results integrate the occurrence of anti-parallel β-strands as unique features of toxic oligomers7-9 with their significant role in the amyloid fibril assembly pathway. These structural insights have implications for the development of therapies and biomarkers.

to appear Nature Communications 2025

An optofluidic antenna for enhancing the sensitivity of single-emitter measurements

Luis Morales-Inostroza, Julian Folz, Ralf Kühnemuth, Suren Felekyan, Franz Wieser, Claus A.M. Seidel, Stephan Götzinger, Vahid Sandoghdar

Many single-molecule investigations are performed in fluidic environments, e.g., to avoid unwanted consequences of contact with surfaces. Diffusion of molecules in this arrangement limits the observation time and the number of collected photons, thus, compromising studies of processes with fast or slow dynamics. Here, we introduce a planar optofluidic antenna (OFA), which enhances the fluorescence signal from molecules by about 5 times per passage, leads to about 7-fold more frequent returns to the observation volume, and significantly lengthens the diffusion time within one passage. We use single-molecule multi-parameter fluorescence detection (sm-MFD), fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) measurements to characterize our OFAs. The antenna advantages are showcased by examining both the slow (ms) and fast (50μs) dynamics of DNA four-way (Holliday) junctions with real-time resolution. The FRET trajectories provide evidence for the absence of an intermediate conformational state and introduce an upper bound for its lifetime. The ease of implementation and compatibility with various microscopy modalities make OFAs broadly applicable to a diverse range of studies.

Exploring the Physics of Basic Medical Research

Vahid Sandoghdar

The 20th century witnessed the emergence of many paradigm-shifting technologies from the physics community, which have revolutionized medical diagnostics and patient care. However, fundamental medical research has been mostly guided by methods from areas such as cell biology, biochemistry, and genetics, with fairly small contributions from physicists. In this Essay, I outline some key phenomena in the human body that are based on physical principles and yet govern our health over a vast range of length and time scales. I advocate that research in life sciences can greatly benefit from the methodology, know-how, and mindset of the physics community and that the pursuit of basic research in medicine is compatible with the mission of physics.<br><br>

invited essay

A paintbrush for delivery of nanoparticles and molecules to live cells with precise spatiotemporal control

Cornelia Holler, Richard W. Taylor, Alexandra Schambony, Leonhard Möckl, Vahid Sandoghdar

Delivery of very small amounts of reagents to the near-field of cells with micrometer spatial precision and millisecond time resolution is currently out of reach. Here we present μkiss as a micropipette-based scheme for brushing a layer of small molecules and nanoparticles onto the live cell membrane from a subfemtoliter confined volume of a perfusion flow. We characterize our system through both experiments and modeling, and find excellent agreement. We demonstrate several applications that benefit from a controlled brush delivery, such as a direct means to quantify local and long-range membrane mobility and organization as well as dynamical probing of intercellular force signaling.

Spectral splitting of a stimulated Raman transition in a single molecule

Johannes Zirkelbach, Burak Gürlek, Masoud Mirzaei, Alexey Shkarin, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

The small cross-section of Raman scattering poses a great challenge for its direct study at the single-molecule level. By exploiting the high Franck-Condon factor of a common-mode resonance, choosing a large vibrational frequency difference in electronic ground and excited states and operating at T<2K, we succeed at driving a coherent stimulated Raman transition in individual molecules. We observe and model a spectral splitting that serves as a characteristic signature of the phenomenon at hand. Our study sets the ground for exploiting the intrinsic optomechanical degrees of freedom of molecules for applications in solid-state quantum optics and information processing.

On-chip interference of scattering from two individual molecules

Dominik Rattenbacher, Alexey Shkarin, Jan Renger, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

Integrated photonic circuits offer a promising route for studying coherent cooperative effects of a controlled collection of quantum emitters. However, spectral inhomogeneities, decoherence, and material incompatibilities in the solid state make this a nontrivial task. Here, we demonstrate efficient coupling of a pair of Fourier-limited organic molecules embedded in a polyethylene film to a TiO2 microdisc resonator on a glass chip. Moreover, we tune the resonance frequencies of the emitters with respect to that of the microresonator by employing nanofabricated electrodes. For two molecules separated by a distance of about 8 µm and an optical phase difference of about pi/2, we report on a large collective extinction of the incident light in the forward direction and the destructive interference of its scattering in the backward direction. Our work sets the ground for coherent coupling of several quantum emitters via a common mode and realization of polymer-based hybrid quantum photonic circuits.

Insights into protein structure using cryogenic light microscopy

Hisham Mazal, Franz Wieser, Vahid Sandoghdar

Fluorescence microscopy has witnessed many clever innovations in the last two decades, leading to new methods such as structured illumination and super-resolution microscopies. The attainable resolution in biological samples is, however, ultimately limited by residual motion within the sample or in the microscope setup. Thus, such experiments are typically performed on chemically fixed samples. Cryogenic light microscopy (Cryo-LM) has been investigated as an alternative, drawing on various preservation techniques developed for cryogenic electron microscopy (Cryo-EM). Moreover, this approach offers a powerful platform for correlative microscopy. Another key advantage of Cryo-LM is the strong reduction in photobleaching at low temperatures, facilitating the collection of orders of magnitude more photons from a single fluorophore. This results in much higher localization precision, leading to Angstrom resolution. In this review, we discuss the general development and progress of Cryo-LM with an emphasis on its application in harnessing structural information on proteins and protein complexes.

Label-free discrimination of extracellular vesicles from large lipoproteins

Anna D. Kashkanova, Martin Blessing, Marie Reischke, Jan-Ole Baur, Andreas S. Baur, Vahid Sandoghdar, Jan Van Deun

Extracellular vesicles (EVs) are increasingly gaining interest as biomarkers and therapeutics. Accurate sizing and quantification of EVs remain problematic, given their nanometre size range and small scattering cross-sections. This is compounded by the fact that common EV isolation methods result in co-isolation of particles with comparable features. Especially in blood plasma, similarly-sized lipoproteins outnumber EVs to a great extent. Recently, interferometric nanoparticle tracking analysis (iNTA) was introduced as a particle analysis method that enables determining the size and refractive index of nanoparticles with high sensitivity and precision. In this work, we apply iNTA to differentiate between EVs and lipoproteins, and compare its performance to conventional nanoparticle tracking analysis (NTA). We show that iNTA can accurately quantify EVs in artificial EV-lipoprotein mixtures and in plasma-derived EV samples of varying complexity. Conventional NTA could not report on EV numbers, as it was not able to distinguish EVs from lipoproteins. iNTA has the potential to become a new standard for label-free EV characterization in suspension.

Quantum Efficiency of Single Dibenzoterrylene Molecules in p-Dichlorobenzene at Cryogenic Temperatures

Mohammad Musavinezhad, Alexey Shkarin, Dominik Rattenbacher, Jan Renger, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

We measure the quantum efficiency (QE) of individual dibenzoterrylene (DBT) molecules embedded in p-dichlorobenzene at cryogenic temperatures. To achieve this, we combine two distinct methods based on the maximal photon emission and on the power required to saturate the zero-phonon line to compensate for uncertainties in some key system parameters. We find that the outcomes of the two approaches are in good agreement for reasonable values of the parameters involved, reporting a large fraction of molecules with QE values above 50%, with some exceeding 70%. Furthermore, we observe no correlation between the observed lower bound on the QE and the lifetime of the molecule, suggesting that most of the molecules have a QE exceeding the established lower bound. This confirms the suitability of DBT for quantum optics experiments. In light of previous reports of low QE values at ambient conditions, our results hint at the possibility of a strong temperature dependence of the QE.

Confocal Interferometric Scattering Microscopy Reveals 3D Nanoscopic Structure and Dynamics in Live Cells

Michelle Küppers, David Albrecht, Anna D. Kashkanova, Jennifer Lühr, Vahid Sandoghdar

Bright-field light microscopy and related techniques continue to play a key role in life sciences because they provide a facile and label-free insight into biological specimen. However, lack of three-dimensional imaging and low sensitivity to nanoscopic features hamper their application in high-end quantitative studies. Here, we remedy these shortcomings by employing confocal interferometric scattering (iSCAT) microscopy. We demonstrate the performance of this label-free technique in a selection of case studies in live cells and benchmark our findings against simultaneously acquired fluorescence images. We reveal the nanometric topography of the nuclear envelope, quantify the dynamics of the endoplasmic reticulum, detect single microtubules, and map nanoscopic diffusion of clathrin-coated pits undergoing endocytosis. Furthermore, we introduce the combination of confocal and wide-field iSCAT modalities for simultaneous imaging of cellular structures and high-speed tracking of nanoscopic entities such as single SARS-CoV2 virions. Confocal iSCAT can be readily implemented as an additional contrast mechanism in existing laser scanning microscopes.

Self-supervised machine learning pushes the sensitivity limit in label-free detection of single proteins below 10 kDa

Mahyar Dahmardeh, Houman Mirzaalian Dastjerdi, Hisham Mazal, Harald Köstler, Vahid Sandoghdar

Interferometric scattering (iSCAT) microscopy is a label-free optical method capable of detecting single proteins, localizing their binding positions with nanometer precision, and measuring their mass. In the ideal case, iSCAT is limited by shot noise such that collection of more photons should extend its detection sensitivity to biomolecules of arbitrarily low mass. However, a number of technical noise sources combined with speckle-like background fluctuations have restricted the detection limit in iSCAT. Here, we show that an unsupervised machine learning isolation forest algorithm for anomaly detection pushes the mass sensitivity limit by a factor of 4 to below 10 kDa. We implement this scheme both with a user-defined feature matrix and a self-supervised FastDVDNet and validate our results with correlative fluorescence images recorded in total internal reflection mode. Our work opens the door to optical investigations of small traces of biomolecules and disease markers such as α-synuclein, chemokines and cytokines.<br><br>

Robust Tipless Positioning Device for Near-Field Investigations: Press and Roll Scan (PROscan)

Hsuan-Wei Liu, Michael A. Becker, Korenobu Matsuzaki, Randhir Kumar, Stephan Götzinger, Vahid Sandoghdar

Scanning probe microscopes scan and manipulate a sharp tip in the immediate vicinity of a sample surface. The limited bandwidth of the feedback mechanism used for stabilizing the separation between the tip and the sample makes the fragile nanoscopic tip very susceptible to mechanical instabilities. We propose, demonstrate, and characterize an alternative device based on bulging a thin substrate against a second substrate and rolling them with respect to each other. We showcase the power of this method by placing gold nanoparticles and semiconductor quantum dots on the two opposite substrates and positioning them with nanometer precision to enhance the fluorescence intensity and emission rate. Furthermore, we exhibit the passive mechanical stability of the system over more than 1 h. Our design concept finds applications in a variety of other scientific and technological contexts, where nanoscopic features have to be positioned and kept near contact with each other. a thin substrate against a second substrate and rolling them with respect each other. We showcase the power of this method by placing gold nanoparticles and semiconductor quantum dots on the two opposite substrates and positioning them with nanometer precision to enhance the fluorescence intensity and emission rate. We exhibit the passive mechanical stability of the system over more than one hour. The design concept presented in this work holds promise in a variety of other contexts, where nanoscopic features have to be positioned and kept near contact with each other.

Deciphering a hexameric protein complex with Angstrom optical resolution

Hisham Mazal, Franz Wieser, Vahid Sandoghdar

Cryogenic optical localization in three dimensions (COLD) was recently shown to resolve up to four binding sites on a single protein. However, because COLD relies on intensity fluctuations that result from the blinking behavior of fluorophores, it is limited to cases where individual emitters show different brightness. This significantly lowers the measurement yield. To extend the number of resolved sites as well as the measurement yield, we employ partial labeling and combine it with polarization encoding in order to identify single fluorophores during their stochastic blinking. We then use a particle classification scheme to identify and resolve heterogenous subsets and combine them to reconstruct the three-dimensional arrangement of large molecular complexes. We showcase this method (polarCOLD) by resolving the trimer arrangement of proliferating cell nuclear antigen (PCNA) and six different sites of the hexamer protein Caseinolytic Peptidase B (ClpB) of Thermus thermophilus in its quaternary structure, both with Angstrom resolution. The combination of polarCOLD and single-particle cryogenic electron microscopy (cryoEM) promises to provide crucial insight into intrinsic heterogeneities of biomolecular structures. Furthermore, our approach is fully compatible with fluorescent protein labeling and can, thus, be used in a wide range of studies in cell and membrane biology.

Precision size and refractive index analysis of weakly scattering nanoparticles in polydispersions

Anna D. Kashkanova, Martin Blessing, André Gemeinhardt, Didier Soulat, Vahid Sandoghdar

Characterization of the size and material properties of particles in liquid suspensions is in very high demand, for example, in the analysis of colloidal samples or of bodily fluids such as urine or blood plasma. However, existing methods are limited in their ability to decipher the constituents of realistic samples. Here we introduce iNTA as a new method that combines interferometric detection of scattering with nanoparticle tracking analysis to reach unprecedented sensitivity and precision in determining the size and refractive index distributions of nanoparticles in suspensions. After benchmarking iNTA with samples of colloidal gold, we present its remarkable ability to resolve the constituents of various multicomponent and polydisperse samples of known origin. Furthermore, we showcase the method by elucidating the refractive index and size distributions of extracellular vesicles from Leishmania parasites and human urine. The current performance of iNTA already enables advances in several important applications, but we also discuss possible improvements.

PiSCAT: A Python Package for Interferometric Scattering Microscopy

Houman Mirzaalian Dastjerdi, Reza Gholami Mahmoodabadi, Matthias Bär, Vahid Sandoghdar, Harald Köstler

Interferometric scattering (iSCAT) microscopy allows one to image and track nano-objects with a nanometer spatial and microsecond temporal resolution over arbitrarily long measurement times (Lindfors et al., 2004; Taylor & Sandoghdar, 2019b, 2019a). A key advantage of this technique over the well-established fluorescence methods is the indefinite photostability of the scattering phenomenon in contrast to the photobleaching of fluorophores. This means that one can perform very long measurements. Moreover, scattering processes are linear and thus do not saturate. This leads to larger signals than is possible from a single fluorophore. As a result, one can image at a much faster rate than in fluorescence microscopy. Furthermore, the higher signal makes it possible to localize a nano-object with much better spatial precision. The remarkable sensitivity of iSCAT, however, also brings about the drawback that one obtains a rich speckle-like background from other nano-objects in the field of view.

High-resolution vibronic spectroscopy of a single molecule embedded in a crystal

Johannes Zirkelbach, Masoud Mirzaei, Irena Deperasińska, Boleslaw Kozankiewicz, Burak Gürlek, Alexey Shkarin, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

Vibrational levels of the electronic ground states in dye molecules have not been previously explored at a high resolution in solid matrices. We present new spectroscopic measurements on single polycyclic aromatic molecules of dibenzoterrylene embedded in an organic crystal made of para-dichlorobenzene. To do this, we use narrow-band continuous-wave lasers and combine spectroscopy methods based on fluorescence excitation and stimulated emission depletion to assess individual vibrational linewidths in the electronic ground state at a resolution of ∼30 MHz dictated by the linewidth of the electronic excited state. In this fashion, we identify several exceptionally narrow vibronic levels with linewidths down to values around 2 GHz. Additionally, we sample the distribution of vibronic wavenumbers, relaxation rates, and Franck–Condon factors, in both the electronic ground and excited states for a handful of individual molecules. We discuss various noteworthy experimental findings and compare them with the outcome of density functional theory calculations. The highly detailed vibronic spectra obtained in our work pave the way for studying the nanoscopic local environment of single molecules. The approach also provides an improved understanding of the vibrational relaxation mechanisms in the electronic ground state, which may help create long-lived vibrational states for applications in quantum technology.

Optimized analysis for sensitive detection and analysis of single proteins via interferometric scattering microscopy

Houman Mirzaalian Dastjerdi, Mahyar Dahmardeh, André Gemeinhardt, Reza Gholami Mahmoodabadi, Harald Köstler, Vahid Sandoghdar

It has been shown that interferometric detection of Rayleigh scattering (iSCAT) can reach an exquisite sensitivity for label-free detection of nano-matter, down to single proteins. The sensitivity of iSCAT detection is intrinsically limited by shot noise, which can be indefinitely improved by employing higher illumination power or longer integration times. In practice, however, a large speckle-like background and technical issues in the experimental setup limit the attainable signal-to-noise ratio. Strategies and algorithms in data analysis are, thus, crucial for extracting quantitative results from weak signals, e.g. regarding the mass (size) of the detected nano-objects or their positions. In this article, we elaborate on some algorithms for processing iSCAT data and identify some key technical as well as conceptual issues that have to be considered when recording and interpreting the data. The discussed methods and analyses are made available in the extensive python-based platform, PiSCAT.

Single-molecule vacuum Rabi splitting: four-wave mixing and optical switching at the single-photon level

André Pscherer, Manuel Meierhofer, Daqing Wang, Hrishikesh Kelkar, Diego-Martin Cano, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

A single quantum emitter can possess a very strong intrinsic nonlinearity, but its overall promise for nonlinear effects is hampered by the challenge of efficient coupling to incident photons. Common nonlinear optical materials, on the other hand, are easy to couple to but are bulky, imposing a severe limitation on the miniaturization of photonic systems. In this work, we show that a single organic molecule acts as an extremely efficient nonlinear optical element in the strong coupling regime of cavity quantum electrodynamics. We report on single-photon sensitivity in nonlinear signal generation and all-optical switching. Our work promotes the use of molecules for applications such as integrated photonic circuits, operating at very low powers.

suggested by editors

Engineering long-lived vibrational states for an organic molecule

Burak Gürlek, Vahid Sandoghdar, Diego-Martin Cano

The optomechanical character of molecules was discovered by Raman about one century ago. Today, molecules are promising contenders for high-performance quantum optomechanical platforms because their small size and large energy-level separations make them intrinsically robust against thermal agitations. Moreover, the precision and throughput of chemical synthesis can ensure a viable route to quantum technological applications. The challenge, however, is that the coupling of molecular vibrations to environmental phonons limits their coherence to picosecond time scales. Here, we improve the optomechanical quality of a molecule by several orders of magnitude through phononic engineering of its surrounding. By dressing a molecule with long-lived high-frequency phonon modes of its nanoscopic environment, we achieve storage and retrieval of photons at millisecond time scales and allow for the emergence of single-photon strong coupling in optomechanics. Our strategy can be extended to the realization of molecular optomechanical networks.

On Quantum Efficiency Measurements and Plasmonic Nano-Antennas

Korenobu Matsuzaki, Hsuan-Wei Liu, Stephan Götzinger, Vahid Sandoghdar

Quantum efficiency is a key quantity that describes the performance of light-emitting materials and is, thus, an important metric for assessing novel nanophotonic systems. This Perspective provides a concise discussion of the difficulties encountered in the characterization of quantum efficiencies, especially for studies that involve single emitters. In particular, we review various approaches that have been recently used for determining quantum efficiencies of emitters coupled to plasmonic antennas and highlight the subtleties and challenges that hinder precise measurements.

Single organic molecules for photonic quantum technologies

C. Toninelli, I. Gerhardt, A.S. Clark, A. Reserbat-Plantey, Stephan Götzinger, Z. Ristanovic, M. Colautti, P. Lombardi, K.D. Major, et al.

Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines, that are tens of megahertz wide, limited only by the excited state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the last decades, the controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single photon sources and as non-linear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.

Nanoscopic charge fluctuations in a gallium phosphide waveguide measured by single molecules

Alexey Shkarin, Dominik Rattenbacher, Jan Renger, Simon Hönl, Tobias Utikal, Paul Seidler, Stephan Götzinger, Vahid Sandoghdar

We present efficient coupling of single organic molecules to a gallium phosphide subwavelengthwaveguide (nanoguide). By examining and correlating the temporal dynamics of various single-molecule resonances at different locations along the nanoguide, we reveal light-induced fluctuationsof their Stark shifts. Our observations are consistent with the predictions of a simple model basedon the optical activation of a small number of charges in the GaP nanostructure.

Precision single-particle localization using radial variance transform

Anna D. Kashkanova, Alexey Shkarin, Reza Gholami Mahmoodabadi, Martin Blessing, Yazgan Tuna, André Gemeinhardt, Vahid Sandoghdar

We introduce an image transform designed to highlight features with high degree of radial symmetry for identification and subpixel localization of particles in microscopy images. The transform is based on analyzing pixel value variations in radial and angular directions. We compare the subpixel localization performance of this algorithm to other common methods based on radial or mirror symmetry (such as fast radial symmetry transform, orientation alignment transform, XCorr, and quadrant interpolation), using both synthetic and experimentally obtained data. We find that in all cases it achieves the same or lower localization error, frequently reaching the theoretical limit.

Polarization-Encoded Colocalization Microscopy at Cryogenic Temperatures

Daniel Böning, Franz Wieser, Vahid Sandoghdar

Super-resolution localization microscopy is based on determining the positions of individual fluorescent markers in a sample. The major challenge in reaching an ever higher localization precision lies in the limited number of collected photons from single emitters. To tackle this issue, it has been shown that one can exploit the increased photostability at low temperatures, reaching localization precisions in the sub-nanometer range. Another crucial ingredient of single-molecule super-resolution imaging is the ability to activate individual emitter within a diffraction-limited spot. Here, we report on photoblinking behavior of organic dyes at low temperature and elaborate on the limitations of this ubiquitous phenomenon for selecting single molecules. We then show that recording the emission polarization not only provides access to the molecular orientation, but it also facilitates the assignment of photons to individual blinking molecules. Furthermore, we employ periodical modulation of the excitation polarization as a robust method to effectively switch fluorophores. We bench mark each approach by resolving two emitters on different DNA origami structures.

Kerker effect, superscattering, and scattering dark states in atomic antennas

Rasoul Alaee Khanghah, Akbar Safari, Vahid Sandoghdar, Robert W. Boyd

We study scattering phenomena such as the Kerker effect, superscattering, and scattering dark states in a subwavelength atomic antenna consisting of atoms with only electric dipole transitions. We show that an atomic antenna can exhibit arbitrarily large or small scattering cross sections depending on the geometry of the structure and the direction of the impinging light. We also demonstrate that atoms with only an electric dipole transition can exhibit a directional radiation pattern with zero backscattering when placed in a certain configuration. This is a special case of a phenomenon known as the Kerker effect, which typically occurs in the presence of both electric and magnetic transitions. Our findings open a pathway to design highly directional emitters, nonradiating sources, and highly scattering objects based on individually controlled atoms.

Differential diffusional properties in loose and tight docking prior to membrane fusion

Agata Witkowska, Susann Spindler, Reza Gholami Mahmoodabadi, Vahid Sandoghdar, Reinhard Jahn

Fusion of biological membranes, although mediated by divergent proteins, is believed to follow a common pathway. It proceeds through distinct steps including docking, merger of proximal leaflets (stalk formation), and formation of a fusion pore. However, the structure of these intermediates is difficult to study due to their short lifetime. Previously, we observed a loosely and tightly docked state preceding leaflet merger using arresting point mutations in SNARE proteins, but the nature of these states remained elusive. Here we used interferometric scattering (iSCAT) microscopy to monitor diffusion of single vesicles across the surface of giant unilamellar vesicles (GUVs). We observed that the diffusion coefficients of arrested vesicles decreased during progression through the intermediate states. Modeling allowed for predicting the number of tethering SNARE complexes upon loose docking and the size of the interacting membrane patches upon tight docking. These results shed new light on the nature of membrane-membrane interactions immediately before fusion.

High-precision protein-tracking with interferometric scattering microscopy

Richard W. Taylor, Cornelia Holler, Reza Gholami Mahmoodabadi, Michelle Küppers, Houman Mirzaalian Dastjerdi, Vasily Zaburdaev, Alexandra Schambony, Vahid Sandoghdar

The mobility of proteins and lipids within the cell, sculpted oftentimes by the organisation of the membrane, reveals a great wealth of information on the function and interaction of these molecules as well as the membrane itself. Single particle tracking has proven to be a vital tool to study the mobility of individual molecules and unravel details of their behaviour. Interferometric scattering (iSCAT) microscopy is an emerging technique well suited for visualising the diffusion of gold nanoparticle-labelled membrane proteins to a spatial and temporal resolution beyond the means of traditional fluorescent labels. We discuss the applicability of interferometric single particle tracking (iSPT) microscopy to investigate the minutia in the motion of a protein through measurements visualising the mobility of the epidermal growth factor receptor in various biological scenarios on the live cell.

Nanostructured alkali-metal vapor cells

Tom F. Cutler, William J. Hamlyn, Jan Renger, Kate A. Whittaker, Danielle Pizzey, Ifan G. Hughes, Vahid Sandoghdar, Charles S. Adams

Atom-light interactions in nano-scale systems hold great promise for novel technologies based on integrated emitters and optical modes. We present the design architecture, construction method,<br>and characterization of an all-glass alkali-metal vapor cell with nanometer-scale internal structure. Our cell has a glue-free design which allows versatile optical access, in particular with high numerical aperture optics. By performing spectroscopy in different illumination and detection schemes, we investigate atomic densities and velocity distributions in various nanoscopic landscapes. We apply a two-photon excitation scheme to atoms confined in one dimension within our cells, achieving a resonance line-width of 32 MHz in a counter-propagating geometry, and 57.5 MHz in a co-propagating geometry. Both of these are considerably narrower than the Doppler width (GHz), and are limited<br>by transit time broadening and velocity selection. We also demonstrate sub-Doppler line-widths for atoms confined in two dimensions to micron-sized channels. Furthermore, we illustrate control over vapor density within our cells through nano-scale confinement alone, which could offer a scalable route towards room-temperature devices with single atoms within an interaction volume. Our design offers a robust platform for miniaturized devices that could easily be combined with integrated<br>photonic circuits.

suggested by editors

Partial cloaking of a gold particle by a single molecule

Johannes Zirkelbach, Benjamin Gmeiner, Jan Renger, Pierre Türschmann, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

Extinction of light by material particles stems from losses incurred by absorption or scattering. The extinction cross section is usually treated as an additive quantity, leading to the exponential laws that govern the macroscopic attenuation of light. In this work, we demonstrate that the extinction cross section of a large gold nanoparticle can be substantially reduced, i.e., the particle becomes<br>more transparent, if a single molecule is placed in its near field. This partial cloaking eect results from a coherent plasmonic interaction between the molecule and the nanoparticle, whereby each of them acts as a nano-antenna to modify the radiative properties of the other.

suggested by editors

Quantum metamaterials with magnetic response at optical frequencies

Rasoul Alaee Khanghah, Burak Gürlek, Mohammad Albooyeh, Diego-Martin Cano, Vahid Sandoghdar

We propose novel quantum antennas and metamaterials with strong magnetic response at optical frequencies. Our design is based on the arrangement of natural atoms with only electric dipole transition moments at distances smaller than a wavelength of light but much larger than their physical size. In particular, we show that an atomic dimer can serve as a magnetic antenna at its antisymmetric mode to enhance the decay rate of a magnetic transition in its vicinity by several orders of magnitude. Furthermore, we study metasurfaces composed of atomic bilayers with and without cavities and show that they can fully reflect the electric and magnetic fields of light, thus, forming nearly perfect electric/magnetic mirrors. The proposed quantum metamaterials can be fabricated with available state-of-the-art technologies and promise several applications both in classical optics and quantum engineering.

suggested by editors

Point spread function in interferometric scattering microscopy (iSCAT). Part I: aberrations in defocusing and axial localization

Reza Gholami Mahmoodabadi, Richard W. Taylor, Martin Kaller, Susann Spindler, Mahdi Mazaheri, Kiarash Kasaian, Vahid Sandoghdar

Interferometric scattering (iSCAT) microscopy is an emerging label-free technique optimized for the sensitive detection of nano-matter. Previous iSCAT studies have approximated the point spread function in iSCAT by a Gaussian intensity distribution. However, recent efforts to track the mobility of nanoparticles in challenging speckle environments and over extended axial ranges has necessitated a quantitative description of the interferometric point spread function (iPSF). We present a robust vectorial diffraction model for the iPSF in tandem with experimental measurements and rigorous FDTD simulations. We examine the iPSF under various imaging scenarios to understand how aberrations due to the experimental configuration encode information about the nanoparticle. We show that the lateral shape of the iPSF can be used to achieve nanometric three-dimensional localization over an extended axial range on the order of 10 µm either by means of a fit to an analytical model or calibration-free unsupervised machine learning. Our results have immediate implications for three-dimensional single particle tracking in complex scattering media.

Molecule-photon interactions in phononic environments

Michael Reitz, Christian Sommer, Burak Gürlek, Vahid Sandoghdar, Diego-Martin Cano, Claudiu Genes

Molecules constitute compact hybrid quantum optical systems that can interface photons, electronic degrees of freedom, localized mechanical vibrations, and phonons. In particular, the strong vibronic interaction between electrons and nuclear motion in a molecule resembles the optomechanical radiation pressure Hamiltonian. While molecular vibrations are often in the ground state even at elevated temperatures, one still needs to get a handle on decoherence channels associated with phonons before an efficient quantum optical network based on optovibrational interactions in solid-state molecular systems could be realized. As a step towards a better understanding of decoherence in phononic environments, we take here an open quantum system approach to the nonequilibrium dynamics of guest molecules embedded in a crystal, identifying regimes of Markovian versus non-Markovian vibrational relaxation. A stochastic treatment, based on quantum Langevin equations, predicts collective vibron-vibron dynamics that resembles processes of sub- and super-radiance for radiative transitions. This in turn leads to the possibility of decoupling intramolecular vibrations from the phononic bath, allowing for enhanced coherence times of collective vibrations. For molecular polaritonics in strongly confined geometries, we also show that the imprint of optovibrational couplings onto the emerging output field results in effective polariton cross-talk rates for finite bath occupancies.

Point spread function in interferometric scattering microscopy (iSCAT). Part I: aberrations in defocusing and axial localization

Reza Gholami Mahmoodabadi, Richard W. Taylor, Martin Kaller, Susann Spindler, Mahdi Mazaheri, Kiarash Kasaian, Vahid Sandoghdar

Interferometric scattering (iSCAT) microscopy is an emerging label-free technique optimized for the sensitive detection of nano-matter. Previous iSCAT studies have approximated the point spread function in iSCAT by a Gaussian intensity distribution. However, recent efforts to track the mobility of nanoparticles in challenging speckle environments and over extended axial ranges has necessitated a quantitative description of the interferometric point spread function (iPSF). We present a robust vectorial diffraction model for the iPSF in tandem with experimental measurements and rigorous FDTD simulations. We examine the iPSF under various imaging scenarios to understand how aberrations due to the experimental configuration encode information about the nanoparticle. We show that the lateral shape of the iPSF can be used to achieve nanometric three-dimensional localization over an extended axial range on the order of 10 µm either by means of a fit to an analytical model or calibration-free unsupervised machine learning. Our results have immediate implications for three-dimensional single particle tracking in complex scattering media.

Sub-nanometer resolution in single-molecule photoluminescence imaging

Ben Yang, Gong Chen, Atif Ghafoor, Yufan Zhang, Yao Zhang, Yang Zhang, Yi Luo, Jinlong Yang, Vahid Sandoghdar, et al.

Ambitions to reach atomic resolution with light have been a major force in shaping nano-optics, whereby a central challenge is achieving highly localized optical fields. A promising approach employs plasmonic nanoantennas, but fluorescence quenching in the vicinity of metallic structures often imposes a strict limit on the attainable spatial resolution, and previous studies have reached only 8 nm resolution in fluorescence mapping. Here, we demonstrate spatially and spectrally resolved photolumines-cence imaging of a single phthalocyanine molecule coupled to nanocavity plasmons in a tunnelling junction with a spatial reso-lution down to ∼8 Å and locally map the molecular exciton energy and linewidth at sub-molecular resolution. This remarkable resolution is achieved through an exquisite nanocavity control, including tip-apex engineering with an atomistic protrusion, quenching management through emitter–metal decoupling and sub-nanometre positioning precision. Our findings provide new routes to optical imaging, spectroscopy and engineering of light–matter interactions at sub-nanometre scales.

Ultrahigh-speed imaging of rotational diffusion on a lipid bilayer

Mahdi Mazaheri, Jens Ehrig, Alexey Shkarin, Vasily Zaburdaev, Vahid Sandoghdar

We studied the rotational and translational diffusion of a single gold nanorod linked to a supported lipid bilayer with ultrahigh temporal resolution of two microseconds. By using a home-built polarization-sensitive dark-field microscope, we recorded particle trajectories with lateral precision of three nanometers and rotational precision of four degrees. The large number of trajectory points in our measurements allows us to characterize the statistics of rotational diffusion with unprecedented detail. Our data show apparent signatures of anomalous diffusion such as sublinear scaling of the mean-squared angular displacement and negative values of angular correlation function at small lag times. However, a careful analysis reveals that these effect stem from the residual noise contributions and confirms normal diffusion. Our experimental approach and observations can be extended to investigate diffusive processes of anisotropic nanoparticles in other fundamental systems such as cellular membranes or other two-dimensional fluids.

Nano-Optics in 2020 ± 20

Vahid Sandoghdar

Ensemble-induced strong light-matter coupling of a single quantum emitter

Stefan Schütz, Johannes Schachenmayer, David Hagenmüller, Gavin K. Brennen, Thomas Volz, Vahid Sandoghdar, Thomas W. Ebbesen, Claudiu Genes, Guido Pupillo

We discuss a technique to strongly couple a single target quantum emitter to a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic ensemble of emitters. A collective coupling of the latter to both the cavity and the target emitter induces strong photon nonlinearities in addition to polariton formation, in contrast to common schemes for ensemble strong coupling. We demonstrate that strong coupling at the level of a single emitter can be engineered via coherent and dissipative dipolar interactions with the ensemble, and provide realistic parameters for a possible implementation with <br>SiV− defects in diamond. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavity-assisted quantum chemistry.

Roadmap on quantum light spectroscopy

Shaul Mukamel, Matthias Freyberger, Wolfgang Schleich, Marco Bellini, Alessandro Zavatta, Gerd Leuchs, Christine Silberhorn, Robert W. Boyd, Luis Lorenzo Sánchez-Soto, et al.

Conventional spectroscopy uses classical light to detect matter properties through the variation<br>of its response with frequencies or time delays. Quantum light opens up new avenues for<br>spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and<br>through the variation of photon statistics by coupling to matter. This Roadmap article focuses on<br>using quantum light as a powerful sensing and spectroscopic tool to reveal novel information<br>about complex molecules that is not accessible by classical light. It aims at bridging the quantum<br>optics and spectroscopy communities which normally have opposite goals: manipulating<br>complex light states with simple matter e.g. qubits versus studying complex molecules with<br>simple classical light, respectively. Articles cover advances in the generation and manipulation<br>of state-of-the-art quantum light sources along with applications to sensing, spectroscopy,<br>imaging and interferometry.

Interferometric Scattering (iSCAT) Microscopy & Related Techniques

Richard W. Taylor, Vahid Sandoghdar

Interferometric scattering (iSCAT) microscopy is a powerful tool for label-free sensitive detection and imaging of nanoparticles to high spatiotemporal resolution. As it was born out of detection principles central to conventional microscopy, we begin by surveying the historical development of the microscope to examine how the exciting possibility for interferometric scattering microscopy with sensitivities sufficient to observe single molecules has become a reality. We discuss the theory of interferometric detection and also issues relevant to achieving a high detection sensitivity and speed. A showcase of numerous applications and avenues of novel research across various disciplines that iSCAT microscopy has opened up is also presented.

Coherent nonlinear optics of quantum emitters in nanophotonic waveguides

Pierre Türschmann, Hanna Le Jeannic, Signe F. Simonsen, Harald Haakh, Stephan Götzinger, Vahid Sandoghdar, Peter Lodahl, Nir Rotenberg

Coherent quantum optics, where the phase of a photon is not scrambled as it interacts with an emitter, lies at the heart of many quantum optical effects and emerging technologies. Solid-state emitters coupled to nanophotonic waveguides are a promising platform for quantum devices, as this element can be integrated into complex photonic chips. Yet, preserving the full coherence properties of the coupled emitter-waveguide system is challenging because of the complex and dynamic electromagnetic landscape found in the solid state. Here, we review progress toward coherent light-matter interactions with solid-state quantum emitters coupled to nanophotonic waveguides. We first lay down the theoretical foundation for coherent and nonlinear light-matter interactions of a two-level system in a quasi-one-dimensional system, and then benchmark experimental realizations. We discuss higher order nonlinearities that arise as a result of the addition of photons of different frequencies, more complex energy level schemes of the emitters, and the coupling of multiple emitters via a shared photonic mode. Throughout, we highlight protocols for applications and novel effects that are based on these coherent interactions, the steps taken toward their realization, and the challenges that remain to be overcome.

Interferometric Scattering Microscopy: Seeing Single Nanoparticles and Molecules via Rayleigh Scattering

Richard W. Taylor, Vahid Sandoghdar

Fluorescence microscopy has been the workhorse for investigating optical phenomena at the nanometer scale but this approach confronts several fundamental limits. As a result, there have been a growing number of activities toward the development of fluorescent-free imaging methods. In this Mini Review, we demonstrate that elastic scattering, the most ubiquitous and oldest optical contrast mechanism, offers excellent opportunities for sensitive detection and imaging of nanoparticles and molecules at very high spatiotemporal resolution. We present interferometric scattering (iSCAT) microscopy as the method of choice, explain its theoretical foundation, discuss its experimental nuances, elaborate on its deep connection to bright-field imaging and other established microscopies, and discuss its promise as well as challenges. A showcase of numerous applications and avenues made possible by iSCAT demonstrates its rapidly growing impact on various disciplines concerned with nanoscopic phenomena.

Coherent coupling of single molecules to on-chip ring resonators

Dominik Rattenbacher, Alexey Shkarin, Jan Renger, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

We report on cryogenic coupling of organic molecules to ring microresonators obtained by looping subwavelength waveguides (nanoguides). We discuss fabrication and characterization of the chip-based nanophotonic elements which yield a resonator finesse in the order of 20 when covered by molecular crystals. Our observed extinction dips from single molecules reach 22%, consistent with an expected enhancement factor of up to 11 for the molecular emission into the nanoguide. Future efforts will aim at efficient coupling of a handful of molecules via their interaction with a ring microresonator mode, setting the ground for the realization of quantum optical cooperative effects.

Electrically driven single-photon superradiance from molecular chains in a plasmonic nanocavity

Yang Luo, Gong Chen, Yang Zhang, Li Zhang, Yunjie Yu, Fanfang Kong, Xiaojun Tian, Yao Zhang, Chongxin Shan, et al.

We demonstrate single-photon superradiance from artificially constructed nonbonded zinc-phthalocyanine molecular chains of up to 12 molecules. We excite the system via electron tunneling in a plasmonic nanocavity and quantitatively investigate the interaction of the localized plasmon with single-exciton superradiant states resulting from dipole-dipole coupling. Dumbbell-like patterns obtained by subnanometer resolved spectroscopic imaging disclose the coherent nature of the coupling associated with superradiant states while second-order photon correlation measurements demonstrate single-photon emission. The combination of spatially resolved spectral measurements with theoretical considerations reveals that nanocavity plasmons dramatically modify the linewidth and intensity of emission from the molecular chains, but they do not dictate the intrinsic coherence of the superradiant states. Our studies shed light on the optical properties of molecular collective states and their interaction with nanoscopically localized plasmons.

Nanoprinting organic molecules at the quantum level

Claudio U. Hail, Christian Höller, Korenobu Matsuzaki, Patrik Rohner, Jan Renger, Vahid Sandoghdar, Dimos Poulikakos, Hadi Eghlidi

Organic compounds present a powerful platform for nanotechnological applications. In particular, molecules suitable for optical functionalities such as single photon generation and energy transfer have great promise for complex nanophotonic circuitry due to their large variety of spectral properties, efficient absorption and emission, and ease of synthesis. Optimal integration, however, calls for control over position and orientation of individual molecules. While various methods have been explored for reaching this regime in the past, none satisfies requirements necessary for practical applications. Here, we present direct non-contact electrohydrodynamic nanoprinting of a countable number of photostable and oriented molecules in a nanocrystal host with subwavelength positioning accuracy. We demonstrate the power of our approach by writing arbitrary patterns and controlled coupling of single molecules to the near field of optical nanostructures. Placement precision, high yield and fabrication facility of our method open many doors for the realization of novel nanophotonic devices.

Interferometric scattering microscopy reveals microsecond nanoscopic protein motion on a live cell membrane

Richard W. Taylor, Reza Gholami Mahmoodabadi, Verena Rauschenberger, Andreas Giessl, Alexandra Schambony, Vahid Sandoghdar

Much of the biological functions of a cell are dictated by the intricate motion of proteins within its membrane over a spatial range of nanometers to tens of micrometers and time intervals of microseconds to minutes. While this rich parameter space is not accessible to fluorescence microscopy, it can be within reach of interferometric scattering (iSCAT) particle tracking. Being sensitive even to single unlabeled proteins, however, iSCAT is easily accompanied by a large speckle-like background, which poses a substantial challenge for its application to cellular imaging. Here, we show that these difficulties can be overcome and demonstrate tracking of transmembrane epidermal growth factor receptors (EGFR) with nanometer precision in all three dimensions at up to microsecond speeds and tens of minutes duration. We provide unprecedented examples of nanoscale motion and confinement in ubiquitous processes such as diffusion in the plasma membrane, transport on filopodia, and endocytosis.

Turning a molecule into a coherent two-level quantum system

Daqing Wang, Hrishikesh Kelkar, Diego-Martin Cano, Dominik Rattenbacher, Alexey Shkarin, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

The use of molecules in quantum optical applications has been hampered by incoherent internal vibrations and other phononic interactions with their environment. Here we show that an organic molecule placed into an optical microcavity behaves as a coherent two-level quantum system. This allows the observation of 99% extinction of a laser beam by a single molecule, saturation with less than 0.5 photons and non-classical generation of few-photons super-bunched light. Furthermore, we demonstrate efficient interaction of the molecule–microcavity system with single photons generated by a second molecule in a distant laboratory. Our achievements represent an important step towards linear and nonlinear quantum photonic circuits based on organic platforms.

Label-Free Imaging of Single Proteins Secreted from Living Cells via iSCAT Microscopy

André Gemeinhardt, Matthew Paul McDonald, Katharina König, Michael Aigner, Andreas Mackensen, Vahid Sandoghdar

We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living cells in real time. In this protocol, we cover the fundamental steps to realize an iSCAT microscope and complement it with additional imaging channels to monitor the viability of a cell under study. Following this, we use the method for real-time detection of single proteins as they are secreted from a living cell which we demonstrate with an immortalized B-cell line (Laz388). Necessary steps concerning the preparation of microscope and sample as well as the analysis of the recorded data are discussed. The video protocol demonstrates that iSCAT microscopy offers a straightforward method to study secretion at the single-molecule level.

High-Speed Microscopy of Diffusion in Pore-Spanning Lipid Membranes

Susann Spindler, Jeremias Sibold, Reza Gholami Mahmoodabadi, Claudia Steinem, Vahid Sandoghdar

Pore-spanning membranes (PSMs) provide a highly attractive model system for investigating fundamental processes in lipid bilayers. We measure and compare lipid diffusion in the supported and suspended regions of PSMs prepared on a microfabricated porous substrate. Although some properties of the suspended regions in PSMs have been characterized using fluorescence studies, it has not been possible to examine the mobility of membrane components on the supported membrane parts. Here, we resolve this issue by employing interferometric scattering microscopy (iSCAT). We study the location-dependent diffusion of DOPE 1,2-dioleoylsn-glycero-3-phosphoethanolamine) lipids (DOPE) labeled with gold nanoparticles in (l,2-dioleoyl-sn-glycero-3-phosphocholine) (DOPC) bilayers prepared on holey silicon nitride substrates that were either (i) oxygen-plasma-treated or (ii) functionalized with gold and 6-mercapto-l-hexanol. For both substrate treatments, diffusion in regions suspended on pores with diameters of 5 mu m is found to be free. In the case of functionalization with gold and 6-mercapto-l-hexanol, similar diffusion coefficients are obtained for both the suspended and the supported regions, whereas for oxygen-plasma-treated surfaces, diffusion is almost 4 times slower in the supported parts of the membranes. We attribute this reduced diffusion on the supported parts in the case of oxygen-plasma-treated surfaces to larger membrane-substrate interactions, which lead to a higher membrane tension in the freestanding membrane parts. Furthermore, we find clear indications for a decrease of the diffusion constant in the freestanding regions away from the pore center. We provide a detailed characterization of the diffusion behavior in these membrane systems and discuss future directions.

Manipulation of Quenching in Nanoantenna–Emitter Systems Enabled by External Detuned Cavities: A Path to Enhance Strong-Coupling

Burak Gürlek, Vahid Sandoghdar, Diego-Martin Cano

We show that a broadband Fabry Perot microcavity can assist an emitter coupled to an off-resonant plasmonic nanoantenna to inhibit the nonradiative channels that affect the quenching of fluorescence. We identify the interference mechanism that creates the necessary enhanced couplings and bandwidth narrowing of the hybrid resonance and show that it can assist entering into the strong coupling regime. Our results provide new possibilities for improving the efficiency of solid-state emitters and accessing diverse realms of photophysics with hybrid structures that can be fabricated using existing technologies.

Visualizing single-cell secretion dynamics with single protein sensitivity

Matthew Paul McDonald, André Gemeinhardt, Katharina König, Marek Piliarik, Stefanie Schaffer, Simon Völkl, Andreas Mackensen, Vahid Sandoghdar

Cellular secretion of proteins into the extracellular environment is an essential mediator of critical biological mechanisms, including cell-to-cell communication, immunological response, targeted delivery, and differentiation. Here, we report a novel methodology that allows for the real-time detection and imaging of single unlabeled proteins that are secreted from individual living cells. This is accomplished via interferometric detection of scattered light (iSCAT) and is demonstrated with Laz388 cells, an Epstein Barr virus (EBV)-transformed B cell line. We find that single Laz388 cells actively secrete IgG antibodies at a rate of the order of 100 molecules per second. Intriguingly, we also find that other proteins and particles spanning ca. 100 kDa-1 MDa are secreted from the Laz388 cells in tandem with IgG antibody release, likely arising from EBV-related viral proteins. The technique is general and, as we show, can also be applied to studying the lysate of a single cell. Our results establish label-free iSCAT imaging as a powerful tool for studying the real-time exchange between cells and their immediate environment with single-protein sensitivity.

Cryogenic optical localization provides 3D protein structure data with Angstrom resolution

Siegfried Weisenburger, Daniel Boening, Benjamin Schomburg, Karin Giller, Stefan Becker, Christian Griesinger, Vahid Sandoghdar

We introduce Cryogenic Optical Localization in 3D (COLD), a method to localize multiple fluorescent sites within a single small protein with Angstrom resolution. We demonstrate COLD by determining the conformational state of the cytosolic Per-ARNT-Sim domain from the histidine kinase CitA of Geobacillus thermodenitnficans and resolving the four biotin sites of streptavidin. COLD provides quantitative 3D information about small- to medium-sized biomolecules on the Angstrom scale and complements other techniques in structural biology.

A single molecule as a high-fidelity photon gun for producing intensity-squeezed light

Xiao-Liu Chu, Stephan Goetzinger, Vahid Sandoghdar

A two-level atom cannot emit more than one photon at a time. As early as the 1980s, this quantum feature was identified as a gateway to 'single-photon sources', where a regular excitation sequence would create a stream of light particles with photon number fluctuations below the shot noise(1). Such an intensity-squeezed beam of light would be desirable for a range of applications, such as quantum imaging, sensing, enhanced precision measurements and information processing(2,3). However, experimental realizations of these sources have been hindered by large losses caused by low photon-collection efficiencies and photophysical shortcomings. By using a planar metallodielectric antenna applied to an organic molecule, we demonstrate the most regular stream of single photons reported to date. The measured intensity fluctuations were limited by our detection efficiency and amounted to 2.2 dB squeezing.

Coherent Coupling of a Single Molecule to a Scanning Fabry-Perot Microcavity

Daqing Wang, Hrishikesh Kelkar, Diego-Martin Cano, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

Organic dye molecules have been used in a great number of scientific and technological applications, but their wider use in quantum optics has been hampered by transitions to short-lived vibrational levels, which limit their coherence properties. To remedy this, one can take advantage of optical resonators. Here, we present the first results on coherent molecule-resonator coupling, where a single polycyclic aromatic hydrocarbon molecule extinguishes 38% of the light entering a microcavity at liquid helium temperature. We also demonstrate fourfold improvement of single-molecule stimulated emission compared to free-space focusing and take first steps for coherent mechanical manipulation of the molecular transition. Our approach of coupling molecules to an open and tunable microcavity with a very low mode volume and moderately low quality factors of the order of 10(3) paves the way for the realization of nonlinear and collective quantum optical effects.

Strong plasmonic enhancement of biexciton emission: controlled coupling of a single quantum dot to a gold nanocone antenna

Korenobu Matsuzaki, Simon Vassant, Hsuan-Wei Liu, Anke Dutschke, Bjoern Hoffmann, Xuewen Chen, Silke Christiansen, Matthew R. Buck, Jennifer A. Hollingsworth, et al.

Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantum dots for optoelectronics applications. However, these higher-order radiative processes are usually quenched in colloidal quantum dots by Auger and other nonradiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantum dots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60 and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics.

Levitated Plasmonic Nanoantennas in an Aqueous Environment

Yazgan Tuna, Ji Tae Kim, Hsuan-Wei Liu, Vahid Sandoghdar

We report on the manipulation of a plasmonic nanoantenna in an aqueous solution using an electrostatic trap created between a glass nanopipette and a substrate. By scanning a trapped gold nanosphere in the near field of a single colloidal quantum dot embedded under the substrate surface, we demonstrate about 8-fold fluorescence enhancement over a lateral full width at half maximum of about 45 nm. We analyze our results with the predictions of numerical electromagnetic simulations under consideration of the electrostatic free energy in the trap. Our approach could find applications in a number of experiments, where plasmonic effects are employed at liquid solid interfaces.

A Single-Emitter Gain Medium for Bright Coherent Radiation from a Plasmonic Nanoresonator

Pu Zhang, Igor Protsenko, Vahid Sandoghdar, Xue-Wen Chen

We propose and demonstrate theoretically bright coherent radiation from a plasmonic nanoresonator powered by a single three-level quantum emitter. By introducing a dual-pump scheme in a Raman configuration for the three-level system, we overcome the fast decay of nanoplasmons and achieve macroscopic accumulation of nanoplasmons on the plasmonic nanoresonator for stimulated emission. We utilize the optical antenna effect for efficient radiation of the nanoplasmons and predict photon emission rates of 100 THz with up to 10 ps duration pulses and GHz repetition rates with the consideration of possible heating issue. We show that the ultrafast nature of the nanoscopic coherent source allows for operation with solid-state emitters at room temperature in the presence of fast dephasing. We provide physical interpretations of the results and discuss their realization and implications for ultracompact integration of optoelectronics.

Chip-Based All-Optical Control of Single Molecules Coherently Coupled to a Nanoguide

Pierre Tuerschmann, Nir Rotenberg, Jan Renger, Irina Harder, Olga Lohse, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

The feasibility of many proposals in nano quantum-optics depends on the efficient coupling of photons to individual quantum emitters, the possibility to control this interaction on demand, and the scalability of the experimental platform. To address these issues, we report on chip-based systems made of one-dimensional subwavelength dielectric waveguides (nanoguides) and polycyclic aromatic hydrocarbon molecules. We discuss the design and fabrication requirements, present data on extinction spectroscopy of single molecules coupled to a nanoguide mode, and show how an external optical beam can switch the propagation of light via a nonlinear optical process. The presented architecture paves the way for the investigation of many-body phenomena and polaritonic states and can be readily extended to more complex geometries for the realization of quantum integrated photonic circuits.

Small slot waveguide rings for on-chip quantum optical circuits

Nir Rotenberg, Pierre Tuerschmann, Harald R. Haakh, Diego-Martin Cano, Stephan Goetzinger, Vahid Sandoghdar

Nanophotonic interfaces between single emitters and light promise to enable new quantum optical technologies. Here, we use a combination of finite element simulations and analytic quantum theory to investigate the interaction of various quantum emitters with slot-waveguide rings. We predict that for rings with radii as small as 1.44 mu m, with a Q-factor of 27,900, near-unity emitter-waveguide coupling efficiencies and emission enhancements on the order of 1300 can be achieved. By tuning the ring geometry or introducing losses, we show that realistic emitter-ring systems can be made to be either weakly or strongly coupled, so that we can observe Rabi oscillations in the decay dynamics even for micron-sized rings. Moreover, we demonstrate that slot waveguide rings can be used to directionally couple emission, again with near-unity efficiency. Our results pave the way for integrated solid-state quantum circuits involving various emitters. (C) 2017 Optical Society of America

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

Hannah Stein, Susann Spindler, Navid Bonakdar, Chun Wang, Vahid Sandoghdar

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of mu m in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

Temporal and spectral properties of quantum light

B. Stiller, U. Seyfarth, G. Leuchs, C. Fabre, V. Sandoghdar, N. Treps, L.F. Cugliandolo

Experimental demonstration of a predictable single photon source with variable photon flux

Aigar Vaigu, Geiland Porrovecchio, Xiao-Liu Chu, Sarah Lindner, Marek Smid, Albert Manninen, Christoph Becher, Vahid Sandoghdar, Stephan Gotzinger, et al.

We present a predictable single-photon source (SPS) based on a silicon vacancy centre in nanodiamond which is optically excited by a pulsed laser. At an excitation rate of 70 MHz the source delivers a photon flux large enough to be measured by a low optical flux detector (LOFD). The directly measured photon flux constitutes an absolute reference. By changing the repetition rate of the pulsed laser, we are able to change the photon flux of our SPS in a controllable way which in turn can act as a reference. The advantage of our method is that it does not require precise knowledge of the source efficiency, but the source is calibrated by the LOFD and can be used for detector responsivity characterizations at the few-photon level.

Experimental realization of an absolute single-photon source based on a single nitrogen vacancy center in a nanodiamond

Beatrice Rodiek, Marco Lopez, Helmuth Hofer, Geiland Porrovecchio, Marek Smid, Xiao-Liu Chu, Stephan Gotzinger, Vahid Sandoghdar, Sarah Lindner, et al.

We report on the experimental realization of an absolute single-photon source based on a single nitrogen vacancy (NV) center in a nanodiamond at room temperature and on the calculation of its absolute spectral photon flux from experimental data. The single-photon source was calibrated with respect to its photon flux and its spectral photon rate density. The photon flux was measured with a low-noise silicon photodiode traceable to the primary standard for optical flux, taking into account the absolute spectral power distribution using a calibrated spectroradiometer. The optical radiant flux is adjustable from 55 fW, which is almost the lowest detection limit for the silicon photodiode, and 75 fW, which is the saturation power of the NV center. These fluxes correspond to total photon flux rates between 190,000 photons per second and 260,000 photons per second, respectively. The single-photon emission purity is indicated by a g((2))(0) value, which is between 0.10 and 0.23, depending on the excitation power. To our knowledge, this is the first single-photon source absolutely calibrated with respect to its absolute optical radiant flux and spectral power distribution, traceable to the corresponding national standards via an unbroken traceability chain. The prospects for its application, e.g., for the detection efficiency calibration of single-photon detectors as well as for use as a standard photon source in the low photon flux regime, are promising. (C) 2017 Optical Society of America

Compartmentalization and Transport in Synthetic Vesicles

Christine Schmitt, Anna H. Lippert, Navid Bonakdar, Vahid Sandoghdar, Lars M. Voll

Nanoscale vesicles have become a popular tool in life sciences. Besides liposomes that are generated from phospholipids of natural origin, polymersomes fabricated of synthetic block copolymers enjoy increasing popularity, as they represent more versatile membrane building blocks that can be selected based on their specific physicochemical properties, such as permeability, stability, or chemical reactivity. In this review, we focus on the application of simple and nested artificial vesicles in synthetic biology. First, we provide an introduction into the utilization of multicompartmented vesosomes as compartmentalized nanoscale bioreactors. In the bottom-up development of protocells from vesicular nanoreactors, the specific exchange of pathway intermediates across compartment boundaries represents a bottleneck for future studies. To date, most compartmented bioreactors rely on unspecific exchange of substrates and products. This is either based on changes in permeability of the coblock polymer shell by physicochemical triggers or by the incorporation of unspecific porin proteins into the vesicle membrane. Since the incorporation of membrane transport proteins into simple and nested artificial vesicles offers the potential for specific exchange of substances between subcompartments, it opens new vistas in the design of protocells. Therefore, we devote the main part of the review to summarize the technical advances in the use of phospholipids and block copolymers for the reconstitution of membrane proteins.

Polaritonic normal-mode splitting and light localization in a one-dimensional nanoguide

Harald R. Haakh, Sanli Faez, Vahid Sandoghdar

We theoretically investigate the interaction of light and a collection of emitters in a subwavelength one-dimensional medium (nanoguide), where enhanced emitter-photon coupling leads to efficient multiple scattering of photons. We show that the spectrum of the transmitted light undergoes normal-mode splitting even though no external cavity resonance is employed. By considering densities much higher than those encountered in cold atom experiments, we study the influence of the near-field dipole coupling and disorder on the resulting complex super-radiant and subradiant polaritonic states. In particular, we provide evidence for the longitudinal localization of light in a one-dimensional open system and provide a polaritonic phase diagram. Our results motivate a number of experiments, where new coherent superposition states of light and matter can be realized in the solid state.

Spectroscopy and microscopy of single molecules in nanoscopic channels: spectral behavior vs. confinement depth

Benjamin Gmeiner, Andreas Maser, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

We perform high-resolution spectroscopy and localization microscopy to study single dye molecules confined to nanoscopic dimensions in one direction. We provide the fabrication details of our nanoscopic glass channels and the procedure for filling them with organic matrices. Optical data on hundreds of molecules in different channel depths show a clear trend from narrow stable lines in deep channels to broader linewidths in ultrathin matrices. In addition, we observe a steady blue shift of the center of the inhomogeneous band as the channels become thinner. Furthermore, we use super-resolution localization microscopy to correlate the positions and orientations of the individual dye molecules with the lateral landscape of the organic matrix, including cracks and strain-induced dislocations. Our results and methodology are useful for a number of studies in various fields such as physical chemistry, solid-state spectroscopy, and quantum nano-optics.

Visualization of lipids and proteins at high spatial and temporal resolution via interferometric scattering (iSCAT) microscopy

Susann Spindler, Jens Ehrig, Katharina Koenig, Tristan Nowak, Marek Piliarik, Hannah E. Stein, Richard W. Taylor, Elisabeth Garanger, Sebastien Lecommandoux, et al.

Microscopy based on the interferometric detection of light scattered from nanoparticles (iSCAT) was introduced in our laboratory more than a decade ago. In this work, we present various capabilities of iSCAT for biological studies by discussing a selection of our recent results. In particular, we show tracking of lipid molecules in supported lipid bilayers (SLBs), tracking of gold nanoparticles with diameters as small as 5 nm and at frame rates close to 1 MHz, 3D tracking of Tat peptide-coated nanoparticles on giant unilamellar vesicles (GUVs), imaging the formation of lipid bilayers, sensing single unlabelled proteins and tracking their motion under electric fields, as well as challenges of studying live cell membranes. These studies set the ground for future quantitative research on dynamic biophysical processes at the nanometer scale.

Visualization and ligand-induced modulation of dopamine receptor dimerization at the single molecule level

Alina Tabor, Siegfried Weisenburger, Ashutosh Banerjee, Nirupam Purkayastha, Jonas M. Kaindl, Harald Huebner, Luxi Wei, Teja W. Groemer, Johannes Kornhuber, et al.

G protein–coupled receptors (GPCRs), including dopamine receptors, represent a group of important pharmacological targets. An increased formation of dopamine receptor D2 homodimers has been suggested to be associated with the pathophysiology of schizophrenia. Selective labeling and ligand-induced modulation of dimerization may therefore allow the investigation of the pathophysiological role of these dimers. Using TIRF microscopy at the single molecule level, transient formation of homodimers of dopamine receptors in the membrane of stably transfected CHO cells has been observed. The equilibrium between dimers and monomers was modulated by the binding of ligands; whereas antagonists showed a ratio that was identical to that of unliganded receptors, agonist-bound D2 receptor-ligand complexes resulted in an increase in dimerization. Addition of bivalent D2 receptor ligands also resulted in a large increase in D2 receptor dimers. A physical interaction between the protomers was confirmed using high resolution cryogenic localization microscopy, with ca. 9 nm between the centers of mass.

Visualization of lipids and proteins at high spatial and temporal resolution via interferometric scattering (iSCAT) microscopy

Susann Spindler, Jens Ehrig, Katharina König, Tristan Nowak, Marek Piliarik, Hannah E. Stein, Richard W. Taylor, Elisabeth Garanger, Sebastien Lecommandoux, et al.

Microscopy based on the interferometric detection of light scattered from nanoparticles (iSCAT) was introduced in our laboratory more than a decade ago. In this work, we present various capabilities of iSCAT for biological studies by discussing a selection of our recent results. In particular, we show tracking of lipid molecules in supported lipid bilayers (SLBs), tracking of gold nanoparticles with diameters as small as 5 nm and at frame rates close to 1 MHz, 3D tracking of Tat peptide-coated nanoparticles on giant unilamellar vesicles (GUVs), imaging the formation of lipid bilayers, sensing single unlabelled proteins and tracking their motion under electric fields, as well as challenges of studying live cell membranes. These studies set the ground for future quantitative research on dynamic biophysical processes at the nanometer scale.

Few-photon coherent nonlinear optics with a single molecule

Andreas Maser, Benjamin Gmeiner, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

The pioneering experiments in linear spectroscopy were performed using flames in the 1800s, but nonlinear optical measurements had to wait until lasers became available in the twentieth century. Because the nonlinear cross-section of materials is very small(1,2), macroscopic bulk samples and pulsed lasers are usually used. Numerous efforts have explored coherent nonlinear signal generation from individual nanoparticles(3-5) or small atomic ensembles(6-8) with millions of atoms. Experiments on a single semiconductor quantum dot have also been reported, albeit with a very small yield(9). Here, we report the coherent nonlinear spectroscopy of a single molecule under continuous-wave single-pass illumination and the switching of a laser beam by on the order of ten pump photons. The sharp molecular transitions and efficient photon-molecule coupling at a tight focus(10) allow for optical switching with less than a handful of pump photons and are thus promising for applications in quantum engineering(11).

Light microscopy: an ongoing contemporary revolution

Siegfried Weisenburger, Vahid Sandoghdar

The optical microscope is one of the oldest scientific instruments that is still used in forefront research. Ernst Abbe's nineteenth century formulation of the resolution limit in microscopy let generations of scientists believe that optical studies of individual molecules and resolving subwavelength structures were not feasible. The Nobel Prize in 2014 for super-resolution fluorescence microscopy marks a clear recognition that the old beliefs have to be revisited. In this article, we present a critical overview of various recent developments in optical microscopy. In addition to the popular super-resolution fluorescence methods, we discuss the prospects of various other techniques and imaging contrasts and consider some of the fundamental and practical challenges that lie ahead.

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate

X. W. Chen, K. G. Lee, H. Eghlidi, Stephan Götzinger, Vahid Sandoghdar

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency. (C)2015 Optical Society of America

Spectroscopic detection of single Pr3+ ions on the H-3(4)-D-1(2) transition

Emanuel Eichhammer, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single Pr3+ ions in yttrium orthosilicate nanocrystals via the H-3(4)-P-3(0) transition at a wavelength of 488 nm. Here we show that individual praseodymium ions can also be detected on the more commonly studied H-3(4)-D-1(2) transition at 606 nm. In addition, we present the first measurements of the second-order autocorrelation function, fluorescence lifetime, and emission spectra of single ions in this system as well as their polarization dependencies on both transitions. Furthermore, we demonstrate that by a proper choice of the crystallite, one can obtain narrower spectral lines and, thus, resolve the hyperfine levels of the excited state. We expect our results to make single-ion spectroscopy accessible to a larger scientific community.

When excitons and plasmons meet: Emerging function through synthesis and assembly

Jennifer A. Hollingsworth, Han Htoon, Andrei Piryatinski, Stephan Goetzinger, Vahid Sandoghdar

To meet the challenge of precise nanoscale arrangement of emitter and plasmonic nanoantenna, synthesis and assembly methods continue to evolve in accuracy and reproducibility. This article reviews some of the many strategies being developed for "soft" chemical approaches to precision integration and assembly. We also discuss investigations of the Purcell effect, emission directionality control, and near-unity collection efficiency of photons, emitter emitter coupling, and higher-order emission processes that have been most deeply explored using individual-emitter- (or several-emitter-) nanoantenna pairs fabricated using traditional lithographic methods or dynamically and controllably manipulated using scanning probe methods. Importantly, these results along with theoretical analyses inspire and motivate continued advancements in large-scale synthesis and assembly. We emphasize assembly approaches that have been used to create nanosemiconductor-nanometal hybrids and, in particular, those that have afforded specific plasmonic effects on excitonic properties. We also review direct-synthesis and chemical-linker strategies to creating discrete, though less spatially extended, semiconductor-metal interactions.

Sensing Nanoparticles with a Cantilever-Based Scannable Optical Cavity of Low Finesse and Sub-lambda(3) Volume

Hrishikesh Kelkar, Daqing Wang, Diego Martin-Cano, Bjoern Hoffmann, Silke Christiansen, Stephan Goetzinger, Vahid Sandoghdar

We report on the realization of an open plane-concave Fabry-Perot resonator with a mode volume below lambda(3) at optical frequencies. We discuss some of the less-common features of this microcavity regime and show that the ultrasmall mode volume allows us to detect cavity resonance shifts induced by single nanoparticles even at quality factors as low as 100. Being based on low-reflectivity micromirrors fabricated on a silicon cantilever, our experimental arrangement provides broadband operation, tunability of the cavity resonance, and lateral scanning. These features are interesting for a range of applications including biochemical sensing, modification of photophysics, and optomechanical studies.

Fabrication and characterization of plasmonic nanocone antennas for strong spontaneous emission enhancement

Bjoern Hoffmann, Simon Vassant, Xue-Wen Chen, Stephan Goetzinger, Vahid Sandoghdar, Silke Christiansen

Plasmonic antennas are attractive nanostructures for a large variety of studies ranging from fundamental aspects of light-matter interactions at the nanoscale to industry-relevant applications such as ultrasensitive sensing, enhanced absorption in solar cells or solar fuel generation. A particularly interesting feature of these antennas is that they can enhance the fluorescence properties of emitters. Theoretical calculations have shown that nanocone antennas provide ideal results, but a high degree of manufacturing precision and control is needed to reach optimal performance. In this study, we report on the fabrication of nanocones with base diameters and heights in the range of 100 nm with variable aspect ratios using focused ion beam milling of sputtered nano-crystalline gold layers. The controlled fabrication process allows us to obtain cones with tailored plasmon resonances. The measured plasmon spectra show very good agreement with finite-difference time-domain calculations. Theoretical investigations predict that these nanocones can enhance the spontaneous emission rate of a quantum emitter by several hundred times while keeping its quantum efficiency above 60%.

High-cooperativity nanofiber laser

Sanli Faez, Pierre Türschmann, Vahid Sandoghdar

Cavity-free efficient coupling between emitters and guided modes is of great<br>current interest for nonlinear quantum optics as well as efficient and scalable<br>quantum information processing. In this work, we extend these activities to the<br>coupling of organic dye molecules to a highly confined mode of a nanofiber,<br>allowing mirrorless and low-threshold laser action in an effective mode volume<br>of less than 100 femtoliters. We model this laser system based on<br>semi-classical rate equations and present an analytic compact form of the laser<br>output intensity. Despite the lack of a cavity structure, we achieve a coupling<br>efficiency of the spontaneous emission to the waveguide mode of 0.07(0.01), in<br>agreement with our calculations. In a further experiment, we also demonstrate<br>the use of a plasmonic nanoparticle as a dispersive output coupler. Our laser<br>architecture is promising for a number of applications in optofluidics and<br>provides a fundamental model system for studying nonresonant feedback<br>stimulated emission.

Scanning-aperture trapping and manipulation of single charged nanoparticles

Ji Tae Kim, Susann Spindler, Vahid Sandoghdar

Although trapping and manipulation of small objects have been of interest for a range of applications and many clever techniques have been devised, new methods are still in great demand for handling different materials and geometries. Here, we report on an electrostatic trap that is created in an aqueous medium between the aperture of a nanopipette and a glass substrate without the need for external potentials. After a thorough characterization of the trapping conditions, we show that we can displace or release a particle at will. Furthermore, we demonstrate trapping and manipulation of nanoparticles and lipid vesicles attached to lipid bilayers, paving the way for controlled studies of forces and diffusion associated with biological membranes. We expect the technique to find interesting applications also in other areas such as optonanofluidics and plasmonics.

Conformational distribution of surface-adsorbed fibronectin molecules explored by single molecule localization microscopy

E. Klotzsch, I. Schoen, J. Ries, A. Renn, V. Sandoghdar, V. Vogel

Adsorbed proteins that promote cell adhesion mediate the response of cells to biomaterials and scaffolds. As proteins undergo conformational changes upon surface adsorption, their functional display may be significantly affected by surface chemistry or solution conditions during the adsorption process. A high-resolution localization microscopy technique is extended here to probe the conformation of individual fibronectin (Fn) molecules at the glass-water interface under physiological buffer conditions. To map distances, four available cysteines located on the modules FnIII(7) and FnIII(15) of dimeric Fn were site-specifically labeled with Cy3B, and their relative positions were determined by stepwise photobleaching with nanometer precision. The four labels on single Fn molecules did not show a uniform or linear arrangement. The distances between label positions were distributed asymmetrically around 33 nm with a tail towards higher distances. Exposure of Fn to denaturing solution conditions during adsorption increased the average distances up to 43 nm for 4 M guanidinium HCl, while changing the solution conditions after the adsorption had no effect, indicating that the observed intra-molecular distances are locked-in during the adsorption process. Also surface coatings of different hydrophobicity altered the conformational distribution, shifting label distances from a median of 24 nm on hydrophilic to 49 nm on hydrophobic surfaces. These results further highlight that the conformation of macromolecules at interfaces depends on the adsorption history. While illustrated here for surface adsorbed Fn, the power of localization-based microscopy extends the repertoire of techniques to characterize biomolecules at interfaces.

Experimental realization of an optical antenna designed for collecting 99% of photons from a quantum emitter

X. -L. Chu, T. J. K. Brenner, X. -W. Chen, Y. Ghosh, J. A. Hollingsworth, V. Sandoghdar, Stephan Götzinger

A light source that emits single photons at well-defined times and into a well-defined mode would be a decisive asset for quantum information processing, quantum metrology, and sub-shot-noise detection of absorption. One of the central challenges in the realization of such a deterministic device based on a single quantum emitter concerns the collection of the photons, which are radiated into a 4 pi solid angle. Here, we present the fabrication and characterization of an optical antenna designed to convert the dipolar radiation of an arbitrarily oriented quantum emitter to a directional beam with more than 99% efficiency. Our approach is extremely versatile and can be used for more efficient detection of nanoscopic emitters ranging from semiconductor quantum dots to dye molecules, color centers, or rare-earth ions in various environments. Having addressed the issue of collection efficiency, we also discuss the photophysical limitations of the existing quantum emitters for the realization of a deterministic single-photon source. (C) 2014 Optical Society of America

Spectroscopic detection and state preparation of a single praseodymium ion in a crystal

Tobias Utikal, Emanuel Eichhammer, L. Petersen, Alois Renn, Stephan Götzinger, Vahid Sandoghdar

The narrow optical transitions and long spin coherence times of rare earth ions in crystals make them desirable for a number of applications ranging from solid-state spectroscopy and laser physics to quantum information processing. However, investigations of these features have not been possible at the single-ion level. Here we show that the combination of cryogenic high-resolution laser spectroscopy with optical microscopy allows one to spectrally select individual praseodymium ions in yttrium orthosilicate. Furthermore, this spectral selectivity makes it possible to resolve neighbouring ions with a spatial precision of the order of 10 nm. In addition to elaborating on the essential experimental steps for achieving this long-sought goal, we demonstrate state preparation and read out of the three ground-state hyperfine levels, which are known to have lifetimes of the order of hundred seconds.

Cryogenic Colocalization Microscopy for Nanometer-Distance Measurements

Siegfried Weisenburger, Bo Jing, Dominik Haenni, Luc Reymond, Benjamin Schuler, Alois Renn, Vahid Sandoghdar

The main limiting factor in spatial resolution of localization microscopy is the number of detected photons. Recently we showed that cryogenic measurements improve the photostability of fluorophores, giving access to Angstrom precision in localization of single molecules. Here, we extend this method to colocalize two fluorophores attached to well-defined positions of a double-stranded DNA. By measuring the separations of the fluorophore pairs prepared at different design positions, we verify the feasibility of cryogenic distance measurement with sub-nanometer accuracy. We discuss the important challenges of our method as well as its potential for further improvement and various applications.

Direct optical sensing of single unlabelled proteins and super-resolution imaging of their binding sites

Marek Piliarik, Vahid Sandoghdar

Detection of single analyte molecules without the use of any label would improve the sensitivity of current biosensors by orders of magnitude to the ultimate graininess of biological matter. Over two decades, scientists have succeeded in pushing the limits of optical detection to single molecules using fluorescence. However, restrictions in photophysics and labelling protocols make this technique less attractive for biosensing. Recently, mechanisms based on vibrational spectroscopy, photothermal detection, plasmonics and microcavities have been explored for fluorescence-free detection of single biomolecules. Here, we show that interferometric detection of scattering (iSCAT) can achieve this goal in a direct and label-free fashion. In particular, we demonstrate detection of cancer marker proteins in buffer solution and in the presence of other abundant proteins. Furthermore, we present super-resolution imaging of protein binding with nanometer localization precision. The ease of iSCAT instrumentation promises a breakthrough for label-free studies of interactions involving proteins and other small biomolecules.

Coherent Interaction of Light and Single Molecules in a Dielectric Nanoguide

Sanli Faez, Pierre Tuerschmann, Harald R. Haakh, Stephan Goetzinger, Vahid Sandoghdar

Many of the currently pursued experiments in quantum optics would greatly benefit from a strong interaction between light and matter. Here, we present a simple new scheme for the efficient coupling of single molecules and photons. A glass capillary with a diameter of 600 nm filled with an organic crystal tightly guides the excitation light and provides a maximum spontaneous emission coupling factor (beta) of 18% for the dye molecules doped in the organic crystal. A combination of extinction, fluorescence excitation, and resonance fluorescence spectroscopy with microscopy provides high-resolution spatio-spectral access to a very large number of single molecules in a linear geometry. We discuss strategies for exploring a range of quantum-optical phenomena, including polaritonic interactions in a mesoscopic ensemble of molecules mediated by a single mode of propagating photons.

Single-molecule optical spectroscopy

Michel Orrit, Taekjip Ha, Vahid Sandoghdar

Tracking Single Particles on Supported Lipid Membranes: Multimobility Diffusion and Nanoscopic Confinement

Chia-Lung Hsieh, Susann Spindler, Jens Ehrig, Vahid Sandoghdar

Supported lipid bilayers have been studied intensively over the past two decades. In this work, we study the diffusion of single gold nanoparticles (GNPs) with diameter of 20 nm attached to GM1 ganglioside or DOPE lipids at different concentrations in supported DOPC bilayers. The indefinite photostability of GNPs combined with the high sensitivity of interferometric scattering microscopy (iSCAT) allows us to achieve 1.9 nm spatial precision at 1 ms temporal resolution, while maintaining long recording times. Our trajectories visualize strong transient confinements within domains as small as 20 nm, and the statistical analysis of the data reveals multiple mobilities and deviations from normal diffusion. We present a detailed analysis of our findings and provide interpretations regarding the effect of the supporting substrate and GM1 clustering. We also comment on the use of high-speed iSCAT for investigating diffusion of lipids, proteins, or viruses in lipid membranes with unprecedented spatial and temporal resolution.

Label-free characterization of biomembranes: from structure to dynamics

Alireza Mashaghi, Samaneh Mashaghi, Ilya Reviakine, Ron M. A. Heeren, Vahid Sandoghdar, Mischa Bonn

We review recent progress in the study of the structure and dynamics of phospholipid membranes and associated proteins, using novel label-free analytical tools. We describe these techniques and illustrate them with examples highlighting current capabilities and limitations. Recent advances in applying such techniques to biological and model membranes for biophysical studies and biosensing applications are presented, and future prospects are discussed.

Receptor Concentration and Diffusivity Control Multivalent Binding of Sv40 to Membrane Bilayers

Oliwia M. Szklarczyk, Nélido González-Segredo, Philipp Kukura, Ariella Oppenheim, Daniel Choquet, Vahid Sandoghdar, Ari Helenius, Ivo F. Sbalzarini, Helge Ewers

Incoming Simian Virus 40 particles bind to their cellular receptor, the glycolipid GM1, in the plasma membrane and thereby induce membrane deformation beneath the virion leading to endocytosis and infection. Efficient membrane deformation depends on receptor lipid structure and the organization of binding sites on the internalizing particle. To determine the role of receptor diffusion, concentration and the number of receptors required for stable binding in this interaction, we analyze the binding of SV40 to GM1 in supported membrane bilayers by computational modeling based on experimental data. We measure the diffusion rates of SV40 virions in solution by fluorescence correlation spectroscopy and of the receptor in bilayers by single molecule tracking. Quartz-crystal microbalance with dissipation (QCM-D) is used to measure binding of SV40 virus-like particles to bilayers containing the viral receptor GM1. We develop a phenomenological stochastic dynamics model calibrated against this data, and use it to investigate the early events of virus attachment to lipid membranes. Our results indicate that SV40 requires at least 4 attached receptors to achieve stable binding. We moreover find that receptor diffusion is essential for the establishment of stable binding over the physiological range of receptor concentrations and that receptor concentration controls the mode of viral motion on the target membrane. Our results provide quantitative insight into the initial events of virus-host interaction at the nanoscopic level.

Cryogenic localization of single molecules with angstrom precision

Siegfried Weisenburger, Jing Bo, Alois Renn, Vahid Sandoghdar

The precision in localizing a molecule is ultimately determined by the number of detected photons, which is in turn limited by photobleaching. Currently, fluorophores can be routinely localized to a few tens of nanometers at room temperature. In this work we demonstrate localization precision better than 3 Angstrom by substantial improvement of the molecular photostability at cryogenic temperatures. We discuss the challenges, solutions and promise of our methodology for high-performance co-localization and super-resolution microscopy.

Measuring three-dimensional interaction potentials using optical interference

Nassir Mojarad, Vahid Sandoghdar, Madhavi Krishnan

We describe the application of three-dimensional (3D) scattering interferometric (iSCAT) imaging to the measurement of spatial interaction potentials for nano-objects in solution. We study electrostatically trapped gold particles in a nanofluidic device and present details on axial particle localization in the presence of a strongly reflecting interface. Our results demonstrate high-speed (similar to kHz) particle tracking with subnanometer localization precision in the axial and average 2.5 nm in the lateral dimension. A comparison of the measured levitation heights of trapped particles with the calculated values for traps of various geometries reveals good agreement. Our work demonstrates that iSCAT imaging delivers label-free, high-speed and accurate 3D tracking of nano-objects conducive to probing weak and long-range interaction potentials in solution. (C) 2013 Optical Society of America

Antennas, quantum optics and near-field microscopy

Vahid Sandoghdar, Mario Agio, Xue-Wen Chen, Stephan Götzinger, Kwang-Geol Lee

The atom is the most elementary constituent of any model that describes the quantum nature of light–matter interaction. Because atoms emit and absorb light at well-defined frequencies, nineteenth century scientists thought of them as collections of harmonically oscillating electric dipole moments or EHDs. In the language of modern physics, the latter represent dipolar transitions among the various quantum mechanical states of an atom.<br><br>In a strict definition, the field of quantum optics deals with problems that not only require the quantization of matter but also of the electromagnetic field, with examples such as (i) generation of squeezed light or Fock states, (ii) strong coupling of an atom and a photon, (iii) entanglement of a photon with an atom and (iv) Casimir and van der Waals forces. There are also many other important topics that have been discussed within the quantum optics community but do not necessarily require a full quantum electrodynamic (QED) treatment. Examples are (i) cooling and trapping of atoms, (ii) precision spectroscopy and (iii) modification of spontaneous emission.<br><br>The simple picture of a TLS as an EHD remains very insightful and valuable to this day. Indeed, much of what we discuss in this chapter has to do with the interplay between the quantum and classical mechanical characters of dipolar oscillators. For instance, the extinction cross-section of a TLS, given by 3λ2/2π, can be derived just as well using quantum mechanics [70] or classical optics [234]. Another example, albeit more subtle, concerns the spontaneous emission rate.

Coherent Interaction of Light with a Metallic Structure Coupled to a Single Quantum Emitter: From Superabsorption to Cloaking

Xue-Wen Chen, Vahid Sandoghdar, Mario Agio

We provide a general theoretical platform based on quantized radiation in absorptive and inhomogeneous media for investigating the coherent interaction of light with material structures in the immediate vicinity of quantum emitters. In the case of a very small metallic cluster, we demonstrate extreme regimes where a single emitter can either counteract or enhance particle absorption by 3 orders of magnitude. For larger structures, we show that an emitter can eliminate both scattering and absorption and cloak a plasmonic antenna. We provide physical interpretations of our results and discuss their applications in active metamaterials and quantum plasmonics. DOI: 10.1103/PhysRevLett.110.153605

Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets

Patrick Galliker, Julian Schneider, Hadi Eghlidi, Sascha Kress, Vahid Sandoghdar, Dimos Poulikakos

Nanotechnology, with its broad impact on societally relevant applications, relies heavily on the availability of accessible nanofabrication methods. Even though a host of such techniques exists, the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters equal to that of a single ejected nanodroplet. The virtually material-independent growth of nanostructures into the third dimension is then governed by an autofocussing phenomenon caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate the capabilities of our electrohydrodynamic printing technique with several examples, including the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.

Spontaneous emission enhancement of a single molecule by a double-sphere nanoantenna across an interface

K-G. Lee, H. Eghlidi, X-W. Chen, A. Renn, S. Goetzinger, V. Sandoghdar

We report on two orders of magnitude reduction in the fluorescence lifetime when a single molecule placed in a thin film is surrounded by two gold nanospheres across the film interface. By attaching one of the gold particles to the end of a glass fiber tip, we could control the modification of the molecular fluorescence at will. We find a good agreement between our experimental data and the outcome of numerical calculations. (C) 2012 Optical Society of America

Metallodielectric Hybrid Antennas for Ultrastrong Enhancement of Spontaneous Emission

Xue-Wen Chen, Mario Agio, Vahid Sandoghdar

We devise new optical antennas that reduce the excited-state radiative lifetimes of emitters to the order of 100 fs while maintaining quantum efficiencies of about 80% at a broadband operation. Here, we combine metallic nanoparticles with planar dielectric structures and exploit design strategies from plasmonic nanoantennas and concepts from cavity quantum electrodynamics to maximize the local density of states and minimize the nonradiative losses incurred by the metallic constituents. The proposed metallodielectric hybrid antennas promise an important impact on various fundamental and applied research fields, including photophysics, ultrafast plasmonics, bright single-photon sources, and Raman spectroscopy.

Single-Photon Spectroscopy of a Single Molecule

Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S. Goetzinger, V. Sandoghdar

Efficient interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single-photon streams with specific wavelengths, bandwidths, and power as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons.

Funneling propagating photons into single molecules

Vahid Sandoghdar

Two decades ago it was commonly believed that single molecules were too small to be seen using an optical microscope. Today, single-molecule fluorescence microscopy has become a standard tool in many biology laboratories. For example, the diffusion or directed motion of proteins labeled with a dye molecule can be detected and tracked by the fluorescent emission of the dye. An advantage of this approach is its specificity; each dye molecule emits at a particular wavelength, enabling the excitation light to be filtered out spectrally. Moreover, one can even count individual photons of fluorescence on a zeroed background. However, organic dye molecules typically used in these processes are prone to photobleaching after ∼1min of illumination. The technique also relies on the high fluorescent quantum yield of the target. Recently, there has been increasing interest in detecting single molecules that do not fluoresce. Here, we image single molecules directly in transmission mode using optics with very high numerical apertures.

Controlling the Phase of a Light Beam with a Single Molecule

M. Pototschnig, Y. Chassagneux, J. Hwang, G. Zumofen, A. Renn, Vahid Sandoghdar

We employ heterodyne interferometry to investigate the effect of a single organic molecule on the phase of a propagating laser beam. We report on the first phase-contrast images of individual molecules and demonstrate a single-molecule electro-optical phase switch by applying a voltage to the microelectrodes embedded in the sample. Our results may find applications in single-molecule holography, fast optical coherent signal processing, and single-emitter quantum operations.

99% efficiency in collecting photons from a single emitter

Xue-Wen Chen, Stephan Goetzinger, Vahid Sandoghdar

In a previous paper [Nat. Photon. 5, 166 ( 2011)], we reported on a planar dielectric antenna that achieved 96% efficiency in collecting the photons emitted by a single molecule. In that work, the transition dipole moment of the molecule was set perpendicular to the antenna plane. Here, we present a theoretical extension of that scheme that reaches collection efficiencies beyond 99% for emitters with arbitrarily oriented dipole moments. Our work opens important doors in a wide range of contexts including quantum optics, quantum metrology, nanoanalytics, and biophysics. In particular, we provide antenna parameters to realize ultrabright single-photon sources in high-index materials such as semiconductor quantum dots and color centers in diamond, as well as sensitive detection of single molecules in nanofluidic devices. (C) 2011 Optical Society of America

A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency

K. G. Lee, X. W. Chen, H. Eghlidi, P. Kukura, R. Lettow, A. Renn, Vahid Sandoghdar, Stephan Götzinger

Single emitters have been considered as sources of single photons in various contexts, including cryptography, quantum computation, spectroscopy and metrology(1-3). The success of these applications will crucially rely on the efficient directional emission of photons into well-defined modes. To accomplish high efficiency, researchers have investigated microcavities at cryogenic temperatures(4,5), photonic nanowires(6,7) and near-field coupling to metallic nano-antennas(8-10). However, despite impressive progress, the existing realizations substantially fall short of unity collection efficiency. Here, we report on a theoretical and experimental study of a dielectric planar antenna, which uses a layered structure to tailor the angular emission of a single oriented molecule. We demonstrate a collection efficiency of 96% using a microscope objective at room temperature and obtain record detection rates of similar to 50 MHz. Our scheme is wavelength-insensitive and can be readily extended to other solid-state emitters such as colour centres(11,12) and semiconductor quantum dots(13,14).

Single-molecule imaging by optical absorption

Michele Celebrano, Philipp Kukura, Alois Renn, Vahid Sandoghdar

To date, optical studies of single molecules at room temperature have relied on the use of materials with high fluorescence quantum yield combined with efficient spectral rejection of background light. To extend single-molecule studies to a much larger pallet of substances that absorb but do not fluoresce, scientists have explored the photothermal effect(1), interferometry(2,3), direct attenuation(4) and stimulated emission(5). Indeed, very recently, three groups have succeeded in achieving single-molecule sensitivity in absorption(6-8). Here, we apply modulation-free transmission measurements known from absorption spectrometers to image single molecules under ambient conditions both in the emissive and strongly quenched states. We arrive at quantitative values for the absorption cross-section of single molecules at different wavelengths and thereby set the ground for single-molecule absorption spectroscopy. Our work has important implications for research ranging from absorption and infrared spectroscopy to sensing of unlabelled proteins at the single-molecule level.

Coherent nonlinear single-molecule microscopy

I. Gerhardt, G. Wrigge, J. Hwang, G. Zumofen, Vahid Sandoghdar

We investigate a nonlinear localization microscopy method based on Rabi oscillations of single emitters. We demonstrate the fundamental working principle of this technique using a cryogenic far-field experiment in which subwavelength features smaller than lambda/10 are obtained. Using Monte Carlo simulations, we show the superior localization accuracy of this method under realistic conditions and a potential for higher acquisition speed or a lower number of required photons as compared to conventional linear schemes. The method can be adapted to other emitters than molecules and allows for the localization of several emitters at different distances to a single measurement pixel.

Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature

C. Toninelli, K. Early, J. Bremi, A. Renn, Stephan Götzinger, Vahid Sandoghdar

We investigate the optical properties of Dibenzoterrylene (DBT) molecules in a spin-coated crystalline film of anthracence. By performing single molecule studies, we show that the dipole moments of the DBT molecules are oriented parallel to the plane of the film. Despite a film thickness of only 20 nm, we observe an exceptional photostability at room temperature and photon count rates around 10 6 per second from a single molecule. These properties together with an emission wavelength around 800 nm make this system attractive for applications in nanophotonics and quantum optics. (C) 2010 Optical Society of America

Spontaneous emission of a nanoscopic emitter in a strongly scattering disordered medium

P. V. Ruijgrok, R. Wuest, A. A. Rebane, A. Renn, Vahid Sandoghdar

Fluorescence lifetimes of nitrogen-vacancy color centers in individual diamond nanocrystals were measured at the interface between a glass substrate and a strongly scattering medium. Comparison of the results with values recorded from the same nanocrystals at the glass-air interface revealed fluctuations of fluorescence lifetimes in the scattering medium. After discussing a range of possible systematic effects, we attribute the observed lengthening of the lifetimes to the reduction of the local density of states. Our approach is very promising for exploring the strong three-dimensional localization of light directly on the microscopic scale. (C) 2010 Optical Society of America

Geometry-induced electrostatic trapping of nanometric objects in a fluid

Madhavi Krishnan, Nassiredin Mojarad, Philipp Kukura, Vahid Sandoghdar

The ability to trap an object-whether a single atom or a macroscopic entity-affects fields as diverse as quantum optics(1), soft condensed-matter physics, biophysics and clinical medicine(2). Many sophisticated methodologies have been developed to counter the randomizing effect of Brownian motion in solution(3-10), but stable trapping of nanometre-sized objects remains challenging(8-10). Optical tweezers are widely used traps, but require sufficiently polarizable objects and thus are unable to manipulate small macromolecules. Confinement of single molecules has been achieved using electrokinetic feedback guided by tracking of a fluorescent label, but photophysical constraints limit the trap stiffness and lifetime(8). Here we show that a fluidic slit with appropriately tailored topography has a spatially modulated electrostatic potential that can trap and levitate charged objects in solution for up to several hours. We illustrate this principle with gold particles, polymer beads and lipid vesicles with diameters of tens of nanometres, which are all trapped without external intervention and independently of their mass and dielectric function. The stiffness and stability of our electrostatic trap is easily tuned by adjusting the system geometry and the ionic strength of the solution, and it lends itself to integration with other manipulation mechanisms. We anticipate that these features will allow its use for contact-free confinement of single proteins and macromolecules, and the sorting and fractionation of nanometre-sized objects or their assembly into high-density arrays.

Quantum Interference of Tunably Indistinguishable Photons from Remote Organic Molecules

R. Lettow, Y. L. A. Rezus, A. Renn, G. Zumofen, E. Ikonen, Stephan Götzinger, Vahid Sandoghdar

We demonstrate two-photon interference using two remote single molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we tune and explore the effect of photon distinguishability. We discuss future improvements on the brightness of single-photon beams, their integration by large numbers on chips, and the extension of our experimental scheme to coupling and entanglement of distant molecules.

Fluorescence Enhancement with the Optical (Bi-) Conical Antenna

Ahmad Mohammadi, Franziska Kaminski, Vahid Sandoghdar, Mario Agio

We investigate the properties of finite gold nanocones as optical antennas for enhancing molecular fluorescence. We compute the modification of the excitation rate, spontaneous emission rate, and quantum efficiency as a function of the nanocone base and length, showing that the maximum field and fluorescence enhancements do not occur for the same nanocone parameters. We compare the results with those for nanorods and nanospheroids and find that nanocones perform better.

Single-Molecule Sensitivity in Optical Absorption at Room Temperature

Philipp Kukura, Michele Celebrano, Alois Renn, Vahid Sandoghdar

Sensitive detection of condensed matter is of utmost importance in fundamental research as well as cutting-edge applications such as molecular analytics and diagnostics. Until very recently, all existing methods for the detection of single molecules at room temperature have required highly efficient fluorophores. Here we demonstrate, for the first time that single molecules can also be detected via standard modulation-free absorption measurements. Our work extends single-molecule detection to a huge class of materials that absorb light but do not fluoresce efficiently.

Nanofocusing radially-polarized beams for high-throughput funneling of optical energy to the near field

Xue-Wen Chen, Vahid Sandoghdar, Mario Agio

We theoretically show that a weakly-focused radially polarized beam can excite surface-plasmon-polaritons in metal nanowires and nanocones with efficiencies of the order of 90% and large bandwidths. The coupling mechanism relies on the formation of a standing wave on the nanowire facet, which imposes a relationship between the operating wavelength and the nanowire radius. An immediate application of this finding is nanofocusing of optical energy for implementations of ultra-fast and high-throughput linear and nonlinear nanoscopies, optical nanolithographies, quantum nano-optics and photochemistry at the nanoscale. (C)2010 Optical Society of America

Efficient coupling of single photons to single plasmons

M. Celebrano, R. Lettow, P. Kukura, M. Agio, A. Renn, Stephan Götzinger, Vahid Sandoghdar

We demonstrate strong coupling of single photons emitted by individual molecules at cryogenic and ambient conditions to individual nanoparticles. We provide images obtained both in transmission and reflection, where an efficiency greater than 55% was achieved in converting incident narrow-band photons to plasmon-polaritons (plasmons) of a silver nanoparticle. Our work paves the way to spectroscopy and microscopy of nano-objects with sub-shot noise beams of light and to triggered generation of single plasmons and electrons in a well-controlled manner. (C) 2010 Optical Society of America

A scanning microcavity for in situ control of single-molecule emission

C. Toninelli, Y. Delley, T. Stoeferle, A. Renn, Stephan Götzinger, Vahid Sandoghdar

We report on the fabrication and characterization of a scannable Fabry-Perot microcavity, consisting of a curved micromirror at the end of an optical fiber and a planar distributed Bragg reflector. Furthermore, we demonstrate the coupling of single organic molecules embedded in a thin film to well-defined resonator modes. We discuss the choice of cavity parameters that will allow sufficiently high Purcell factors for enhancing the zero-phonon transition between the vibrational ground levels of the electronic excited and ground states. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456559]

Coherent state preparation and observation of Rabi oscillations in a single molecule

I. Gerhardt, G. Wrigge, G. Zumofen, J. Hwang, A. Renn, Vahid Sandoghdar

We report on the excitation of single molecules via narrow zero-phonon transitions using short laser pulses. By monitoring the Stokes-shifted fluorescence, we studied the excited state population as a function of the delay time, laser intensity, and frequency detuning. A pi-pulse excitation was demonstrated with merely 500 photons, and 5 Rabi cycles were achieved at higher excitation powers. Our findings are in good agreement with theoretical calculations and provide a first step toward coherent manipulation of the electronic states of single molecules with few photons.

Lifetime-limited zero-phonon spectra of single molecules in methyl methacrylate

A. Walser, A. Renn, Stephan Götzinger, Vahid Sandoghdar

We report on high resolution single molecule spectroscopy in frozen methyl methacrylate (MMA). We show that the zero-phonon transitions of single dibenzanthanthrene molecules in this polar matrix can reach their natural linewidth limit at T = 1.4 K. Our X-ray diffraction measurements and direct study of single molecule dipole orientation provide clear evidence for the crystalline nature of MMA at low temperatures. Our results hold promise for the controlled study of the transition between crystalline and amorphous matrices, and have implications on cryogenic single molecule microscopy in biological applications. (C) 2009 Elsevier B. V. All rights reserved.

High-speed nanoscopic tracking of the position and orientation of a single virus

Philipp Kukura, Helge Ewers, Christian Mueller, Alois Renn, Ari Helenius, Vahid Sandoghdar

Optical studies have revealed that, after binding, virions move laterally on the plasma membrane, but the complexity of the cellular environment and the drawbacks of fluorescence microscopy have prevented access to the molecular dynamics of early virus-host couplings, which are important for cell infection. Here we present a colocalization methodology that combines scattering interferometry and single-molecule fluorescence microscopy to visualize both position and orientation of single quantum dot-labeled Simian virus 40 (SV40) particles. By achieving nanometer spatial and 8 ms temporal resolution, we observed sliding and tumbling motions during rapid lateral diffusion on supported lipid bilayers, and repeated back and forth rocking between nanoscopic regions separated by 9 nm. Our findings suggest recurrent swap of receptors and viral pentamers as well as receptor aggregation in nanodomains. We discuss the prospects of our technique for studying virus-membrane interactions and for resolving nanoscopic dynamics of individual biological nano-objects.

Resolution and Enhancement in Nanoantenna-Based Fluorescence Microscopy

Hadi Eghlidi, Kwang Geol Lee, Xue-Wen Chen, Stephan Götzinger, Vahid Sandoghdar

Single gold nanoparticles can act as nanoantennas for enhancing the fluorescence of emitters in their near fields. Here we present experimental and theoretical studies of scanning antenna-based fluorescence microscopy as a function of the diameter of the gold nanoparticle. We examine the interplay between fluorescence enhancement and spatial resolution and discuss the requirements for deciphering single molecules in a dense sample. Resolutions better than 20 nm and fluorescence enhancement up to 30 times are demonstrated experimentally. By accounting for the tip shaft and the sample interface in finite-difference time-domain calculations, we explain why the measured fluorescence enhancements are higher in the presence of an interface than the values predicted for a homogeneous environment.

A single-molecule optical transistor

J. Hwang, M. Pototschnig, R. Lettow, G. Zumofen, A. Renn, Stephan Götzinger, Vahid Sandoghdar

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms(1-9). Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nano-metre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams(7). However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities(10-13) or waveguides(8,14). Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second 'gating' beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Spectral dynamics and spatial localization of single molecules in a polymer

A. Walser, G. Zumofen, A. Renn, Stephan Götzinger, Vahid Sandoghdar

We report on the high-resolution spectroscopy of single dibenzanthanthrene molecules embedded in polymethyl methacrylate (PMMA). We employed three methods for the characterization of spectral line shapes based on fitting a Lorentzian function, determining full widths at half-maxima, and calculation of the second-order spectral cumulant. The three approaches provide comparable histograms of linewidth distributions, displaying slowly decaying tails that are indicative of the Levy stable law. In addition, we introduce an alternative method for the analysis of spectral dynamics, in which ensemble spectra are reconstructed by adding single molecule spectral autocorrelations. Furthermore, we examine the spectral width and distributions of single molecules on the PMMA chain length over three orders of magnitude and find a very small dependence. Lastly, we demonstrate that, despite the strong spectral dynamics, it is possible to collect enough photons from single molecules to localize their positions to better than 10 nm.

Circular Grating Resonators as Small Mode-Volume Microcavities for Switching

Sophie Schoenenberger, Nikolaj Moll, Thilo Stoeferle, Rainer F. Mahrt, Bert J. Offrein, Stephan Götzinger, Vahid Sandoghdar, Jens Bolten, Thorsten Wahlbrink, et al.

We demonstrate the suitability of microcavities based on circular grating resonators (CGRs) as fast switches. This type of optical resonator is characterized by a high quality factor and very small mode volume. The waveguide-coupled CGRs are fabricated with silicon-on-insulator technology compatible with standard complementary metal-oxide semiconductor (CMOS) processing. The linear optical properties of the CGRs are investigated by transmission spectroscopy. From 3D finite-difference time-domain simulations of isolated CGRs, we identify the measured resonances. We probe the spatial distribution and the parasitic losses of a resonant optical mode with scanning near-field optical microscopy. We observe fast all-optical switching within a few picoseconds by optically generating free charge carriers within the cavity. (C) 2009 Optical Society of America

Control and imaging of single-molecule spectral dynamics using a nano-electrode

I. Gerhardt, G. Wrigge, Vahid Sandoghdar

We study the influence of a scanning nano-electrode on the fluorescence excitation spectra of single terrylene molecules embedded in thin p-terphenyl films at cryogenic temperatures. We show that applied voltages of less than 10 V can result in reversible Stark shifts of up to 100 times and a linewidth increase greater than 10 times the natural linewidth. We discuss the potential of our experimental scheme for the direct imaging of individual two-level systems in the nanometer vicinity of single molecules.

Seeing diamond defects

Vahid Sandoghdar

Highly Efficient Interfacing of Guided Plasmons and Photons in Nanowires

Xue-Wen Chen, Vahid Sandoghdar, Mario Agio

Successful exploitations of strongly confined surface plasmon-polaritons critically rely on their efficient and rapid conversion to lossless channels. We demonstrate a simple, robust, and broadband butt-coupling technique for connecting a metallic nanowire and a dielectric nanofiber. Conversion efficiencies above 95% in the visible and close to 100% in the near-infrared can be achieved with realistic parameters. Moreover, by combining butt-coupling with nanofocusing, we propose a broadband high-throughput near-field optical microscope.

Metal nanoparticles in strongly confined beams: transmission, reflection and absorption

Nassiredin M. Mojarad, Gert Zumofen, Vahid Sandoghdar, Mario Agio

We investigate the interaction of tightly focused light with the surface-plasmon-polariton resonances of metal nanospheres. In particular, we compute the scattering and absorption ratios as well as transmission and reflection coefficients. Inspired by our previous work in [1], we discuss how well a metal nanoparticle approximates a point-like dipolar radiator. We find that a 100 nm silver nanosphere is very close to such an ideal oscillator. Our results have immediate implications for single nanoparticle spectroscopy and microscopy as well as plasmonics. [DOI: 10.2971/jeos.2009.09014]

Molecules as sources for indistinguishable single photons

Ville Ahtee, Robert Lettow, Robert Pfab, Alois Renn, Erkki Ikonen, Stephan Götzinger, Vahid Sandoghdar

We report on the triggered generation of identical photons by solid-state single-photon sources in two separate cryogenic laser scanning microscopes. Organic fluorescent molecules were used as emitters and investigated by means of high resolution laser spectroscopy. Continuous-wave photon correlation measurements on individual molecules proved the isolation of single quantum systems. By using frequency selective pulsed excitation of the molecule and efficient spectral filtering of its emission, we produced triggered Fourier-limited single photons. In a further step, local electric fields were applied to match the emission wavelengths of two different molecules via Stark effect. Identical single photons are indispensable for the realization of various quantum information processing schemes proposed. The solid-state approach presented here paves the way to the integration of multiple bright sources of single photons on a single chip.

Imaging a Single Quantum Dot When It Is Dark

P. Kukura, M. Celebrano, A. Renn, Vahid Sandoghdar

We have succeeded in recording extinction images of individual cadmium selenide quantum dots at ambient condition. This is achieved by optimizing the interference between the light that is coherently scattered from the quantum dot and the reflection of the incident laser beam. The ability to interrogate the dot in the absence of fluorescence has revealed that its extinction cross section diminishes in the photobleached state, but interestingly, it remains unchanged during fluorescence blinking off times. Our methodology makes optical imaging and spectroscopy accessible to the study of ultrasmall nanoscopic objects such as nonfluorescent macromolecules and single emitters with very low quantum efficiencies.

Gold, Copper, Silver and Aluminum Nanoantennas to Enhance Spontaneous Emission

A. Mohammadi, Vahid Sandoghdar, M. Agio

We compute the decay rates of emitters coupled to spheroidal nanoantennas made of gold, copper, silver, and aluminum. The spectral position of the localized surface plasmon-polariton resonance, the enhancement factors and the quantum efficiency are investigated as a function of the aspect ratio, background index and the metal composing the nanoantenna. While copper yields results similar to gold, silver and aluminum exhibit different performances. Our results show that with a careful choice of the parameters these nanoantennas can enhance emitters ranging from the UV to the near-IR spectrum.

Pushing Optical Microscopy to the Limit: From Single‐Molecule Fluorescence Microscopy to Label‐Free Detection and Tracking of Biological Nano‐Objects

Philipp Kukura, Alois Renn, Vahid Sandoghdar

The promise and success of current biomedical research is highly dependent on understanding the mechanisms of fundamental biological processes such as protein folding, the function of molecular machines, or the interaction of proteins with molecular complexes. In particular, on the cellular level the heterogeneous and fluctuating environment as well as the intrinsic asynchronicity of protein function requires observation of individual copies rather than ensembles if a detailed understanding of their function is to be obtained. As a consequence, real‐time in vivo imaging is one of the most desirable tools for investigating biological systems at microscopic and nanoscopic levels. In this chapter, we summarize some of the recent advances in optical microscopy with a particular focus on the capabilities and limitations of studies at the single‐molecule and nanoparticle level. We present a detailed account of the intricate interplay between single‐molecule photophysical properties and the attainable signal‐to‐noise ratio in the ability to see, localize and trace single molecules. Furthermore, we discuss the possibility of single‐molecule labels being replaced by tiny light scatterers, such as gold nanoparticles, which offer an inherent photostability and lack of saturation. In particular, we introduce a novel interferometric technique for detecting nanoscopic objects and demonstrate its potential use both for the study of gold nanoparticle labels in scattering media and the label‐free detection of single biological nanoparticles in the absence of additional scatterers.

Spheroidal nanoparticles as nanoantennas for fluorescence enhancement

A. Mohammadi, F. Kaminski, Vahid Sandoghdar, M. Agio

Nanoantennas made of spheroidal metal nanoparticles are studied as a function of several parameters, namely aspect ratio, volume, background index and metal. Single nanospheroids are analysed using the polarisability theory with radiative and depolarisation corrections, while double spheroids are investigated using the finite-difference time-domain method. We focus on the spectral position of the plasmon resonance and on the scattering efficiency for applications related to the fluorescence enhancement of emitters. We show that a careful choice of the parameters allows covering wavelengths ranging from the UV to the near IR spectrum, while keeping the scattering efficiency close to 100%. We also discuss the role of the optical constants in determining the nanoantenna performances.

Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence

G. Wrigge, I. Gerhardt, J. Hwang, G. Zumofen, Vahid Sandoghdar

Single dye molecules at cryogenic temperatures exhibit many spectroscopic phenomena known from the study of free atoms and are thus promising candidates for experiments in fundamental quantum optics. However, the existing techniques for their detection have either sacrificed information on the coherence of the excited state or have been ineffcient. Here, we show that these problems can be addressed by focusing the excitation light near to the extinction cross-section of a molecule. Our detection scheme enables us to explore resonance fluorescence over nine orders of magnitude of excitation intensity and to separate its coherent and incoherent parts. In the strong excitation regime, we demonstrate the first direct observation of the Mollow fluorescence triplet from a single solid-state emitter. Under weak excitation, we report the detection of a single molecule with an incident power as faint as 600aW, paving the way for studying nonlinear effects with only a few photons.

Modification of single molecule fluorescence close to a nanostructure: radiation pattern, spontaneous emission and quenching

S. Kuehn, G. Mori, M. Agio, Vahid Sandoghdar

The coupling of nanostructures with emitters opens up ways for the realization of man-made subwavelength light emitting elements. In this article we present an overview of our recent progress in the modification of fluorescence when an emitter is placed close to a nanostructure. In order to control the wealth of parameters that contribute to this process, we have combined scanning probe technology with single molecule microscopy and spectroscopy. We discuss the enhancement and reduction of molecular excitation and emission rates in the presence of a dielectric or metallic nanoparticle and emphasize the role of plasmon resonances in the latter. Furthermore, we examine the spectral and angular emission characteristics of the molecule-particle system. The experimental findings are in excellent semi-quantitative agreement with the outcome of theoretical calculations. We show that the interaction of an emitter with a nanoparticle can be expressed in the framework of an optical nanoantennae and propose arrangements that could lead to the modification of spontaneous emission by more than 1000 times.

Gold nanorods and nanospheroids for enhancing spontaneous emission

A. Mohammadi, Vahid Sandoghdar, M. Agio

We compute the radiative decay rate and the quantum efficiency for an emitter coupled to gold nanorods and nanospheroids using the body-of-revolution finite-difference time-domain method. We study these quantities as a function of the nanoparticle aspect ratio and volume, showing that large enhancements can be achieved with realistic parameters. Moreover, we find that nanospheroids exhibit better performances than nanorods for applications in the visible and near-infrared spectral range.

Exploring the limits of single emitter detection in fluorescence and extinction

G. Wrigge, J. Hwang, I. Gerhardt, G. Zumofen, Vahid Sandoghdar

We present an experimental comparison and a theoretical analysis of the signal-to-noise ratios in fluorescence and extinction spectroscopy of a single emitter. We show that because of its homodyne nature the extinction measurements can be advantageous if the emitter is weakly excited. Furthermore, we discuss the potential of this method for the detection and spectroscopy of weakly emitting systems such as rare earth ions. (C) 2008 Optical Society of America

Coupling of plasmonic nanoparticles to their environments in the context of van der Waals-Casimir interactions

U. Hakanson, M. Agio, S. Kuehn, L. Rogobete, T. Kalkbrenner, Vahid Sandoghdar

We present experiments in which the interaction of a single gold nanoparticle with glass substrates or with another gold particle can be tuned by in situ control of their separations using scanning probe technology. We record the plasmon resonances of the coupled systems as a function of the polarization of the incident field and the particle position. The distinct spectral changes of the scattered light from the particle pair are in good agreement with the outcome of finite-difference time-domain calculations. We believe that our experimental technique holds promise for the investigation of the van der Waals-Casimir-type interactions between nanoscopic neutral bodies.

Plasmon spectra of nanospheres under a tightly focused beam

Nassiredin M. Mojarad, Vahid Sandoghdar, Mario Agio

We study the modification of the far-field cross sections and the near-field enhancement for gold and silver nanospheres illuminated by a tightly focused beam. Using a multipole-expansion approach we obtain an analytical solution to the scattering problem and provide insight on the effects of focusing on the optical response. Large differences with respect to Mie theory are especially found when the nanoparticle supports quadrupole or higher-order resonances. (C) 2008 Optical Society of America.

Perfect Reflection of Light by an Oscillating Dipole

G. Zumofen, N. M. Mojarad, Vahid Sandoghdar, M. Agio

We show theoretically that a directional dipole wave can be perfectly reflected by a single pointlike oscillating dipole. Furthermore, we find that, in the case of a strongly focused plane wave, up to 85% of the incident light can be reflected by the dipole. Our results hold for the full spectrum of the electromagnetic interactions and have immediate implications for achieving strong coupling between a single propagating photon and a single quantum emitter.

Scanning near-field optical coherent spectroscopy of single molecules at 1.4 K

Ilja Gerhardt, Gert Wrigge, Mario Agio, Pavel Bushev, Gert Zumofen, Vahid Sandoghdar

We present scanning near-field extinction spectra of single molecules embedded in a solid matrix. By varying the tip-molecule separation, we modify the line shape of the spectra, demonstrating the coherent nature of the interaction between the incident laser light and the excited state of the molecule. We compare the measured data with the outcome of numerical calculations and find a very good agreement. (c) 2007 Optical Society of America.

Design of plasmonic nanoantennae for enhancing spontaneous emission

Lavinia Rogobete, Franziska Kaminski, Mario Agio, Vahid Sandoghdar

We apply two- and three-dimensional numerical calculations to study optical nanoantennae made of two coupled gold nanostructures, enclosing a single emitter in their gap. We show that, using structures manufacturable with today's nanotechnology, it is possible to increase the radiative decay rate by three orders of magnitude while keeping a quantum efficiency larger than 80% in the near-infrared regime. We examine the competition between the radiative and nonradiative processes in the presence of the antennae as a function of wavelength and antenna geometry. Our results hold great promise for improving the quantum efficiency of poor emitters such as silicon nanocrystals or carbon nanotubes. (c) 2007 Optical Society of America.

Near-field imaging and frequency tuning of a high-Q photonic crystal membrane microcavity

S. Mujumdar, A. F. Koenderink, T. Suenner, B. C. Buchler, M. Kamp, A. Forchel, Vahid Sandoghdar

We discuss experimental studies of the interaction between a nanoscopic object and a photonic crystal membrane resonator of quality factor Q=55000. By controlled actuation of a glass fiber tip in the near field of the photonic crystal, we constructed a complete spatio-spectral map of the resonator mode and its coupling with the fiber tip. On the one hand, our findings demonstrate that scanning probes can profoundly influence the optical characteristics and the near-field images of photonic devices. On the other hand, we show that the introduction of a nanoscopic object provides a low loss method for on-command tuning of a photonic crystal resonator frequency. Our results are in a very good agreement with the predictions of a combined numerical/analytical theory. (C) 2007 Optical Society of America.

Label-free optical detection and tracking of single virions bound to their receptors in supported membrane bilayers

Helge Ewers, Volker Jacobsen, Enrico Klotzsch, Alicia E. Smith, Ari Helenius, Vahid Sandoghdar

We apply an interferometric optical detection scheme to image and track unlabeled single virions. Individual simian virus 40 virions and uninfectious virus-like particles were imaged on a glass substrate and on a supported membrane bilayer. Moreover, single unlabeled virions were tracked when bound to supported membrane bilayers via the viral receptor, the glycolipid GM1. The technology presented here promises to be generally applicable to studying the motion of unlabeled macromolecules on membranes.

Strong extinction of a laser beam by a single molecule

I. Gerhardt, G. Wrigge, P. Bushev, G. Zumofen, M. Agio, R. Pfab, Vahid Sandoghdar

We present an experiment where a single molecule strongly affects the amplitude and phase of a laser field emerging from a subwavelength aperture. We achieve a visibility of -6% in direct and +10% in cross-polarized detection schemes. Our analysis shows that a close to full extinction should be possible using near-field excitation.

Nanoparticle-induced fluorescence lifetime modification as nanoscopic ruler: Demonstration at the single molecule level

J. Seelig, K. Leslie, A. Renn, S. Kuhn, V. Jacobsen, M. van de Corput, C. Wyman, Vahid Sandoghdar

We combine interferometric detection of single gold nanoparticles, single molecule microscopy, and fluorescence lifetime measurement to study the modification of the fluorescence decay rate of an emitter close to a nanoparticle. In our experiment, gold particles with a diameter of 15 nm were attached to single dye molecules via double-stranded DNA of different lengths. Nanoparticle-induced lifetime modification (NPILM) has promise in serving as a nanoscopic ruler for the distance range well beyond 10 nm, which is the upper limit of fluorescence resonant energy transfer (FRET). Furthermore, the simultaneous detection of single nanoparticles and fluorescent molecules presented in this work provides new opportunities for single molecule biophysical studies.

Linear and non-linear optical experiments based on macroporous silicon photonic crystals

Ralf B. Wehrspohn, Stefan L. Schweizer, Vahid Sandoghdar

Macroporous silicon is a model system for 2D silicon photonic crystals. In this review, we describe our recent theoretical and experimental advances in the macroporous silicon technology as well as in novel near-field optical experiments concerning modified light emission from photonic crystal cavities and waveguides. We also review our recent work on tuning silicon photonic crystals by free-carriers or nonlinear effects and compare the two tuning mechanisms. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nano-optomechanical characterization and manipulation of photonic crystals

Sushil Mujumdar, A. Femius Koenderink, Robert Wuest, Vahid Sandoghdar

We describe the application of scanning near-field optical microscopy (SNOM) for the high-resolution visualization of light propagation in photonic crystal structures. We also demonstrate that nanoscopic elements such as sharp tips could be used for the mechanical manipulation of the optical properties of photonic crystals. In particular, our theoretical and experimental results show that narrow resonances of a photonic crystal cavity can be tuned without a substantial influence on its quality factor. Furthermore, we discuss the modification of the fluorescence of a nanoscopic emitter as a function of its location close to a photonic crystal.

Realization of two Fourier-limited solid-state single-photon sources

R. Lettow, V. Ahtee, R. Pfab, A. Renn, E. Ikonen, Stephan Götzinger, Vahid Sandoghdar

We demonstrate two solid-state sources of indistinguishable single photons. High resolution laser spectroscopy and optical microscopy were combined at T = 1.4 K to identify individual molecules in two independent microscopes. The Stark effect was exploited to shift the transition frequency of a given molecule and thus obtain single photon sources with perfect spectral overlap. Our experimental arrangement sets the ground for the realization of various quantum interference and information processing experiments. (c) 2007 Optical Society of America.

Controlled coupling of counterpropagating whispering-gallery modes by a single Rayleigh scatterer: A classical problem in a quantum optical light

A. Mazzei, Stephan Götzinger, L. de S. Menezes, G. Zumofen, O. Benson, Vahid Sandoghdar

We present experiments where a single subwavelength scatterer is used to examine and control the backscattering induced coupling between counterpropagating high-Q modes of a microsphere resonator. Our measurements reveal the standing wave character of the resulting symmetric and antisymmetric eigenmodes, their unbalanced intensity distributions, and the coherent nature of their coupling. We discuss our findings and the underlying classical physics in the framework common to quantum optics and provide a particularly intuitive explanation of the central processes.

Finite-difference time-domain modeling of decay rates in the near field of metal nanostructures

F. Kaminski, Vahid Sandoghdar, M. Agio

We test the three-dimensional finite-difference time-domain method for the calculation of decay rates of an emitter placed in close vicinity to metal nanostructures. By choosing analytical solvable systems we asses its accuracy and indicate the issues deriving from near-field coupling, Ohmic losses and staircasing. The latter is found to be the most problematic one, because it gives rise to spurious peaks that survive even for very fine discretizations.

Controlled photon transfer between two individual nanoemitters via shared high-Q modes of a microsphere resonator

Stephan Götzinger, L. de S. Menezes, A. Mazzei, S. Kuhn, Vahid Sandoghdar, O. Benson

We realize controlled cavity-mediated photon transfer between two single nanoparticles over a distance of several tens of micrometers. First, we show how a single nanoscopic emitter attached to a near-field probe can be coupled to high-Q whispering-gallery modes of a silica microsphere at will. Then we demonstrate transfer of energy between this and a second nanoparticle deposited on the sphere surface. We estimate the photon transfer efficiency to be about 6 orders of magnitude higher than that via free-space propagation at comparable separations.

Oxygen-dependent photochemistry of fluorescent dyes studied at the single molecule level

Alois Renn, Johannes Seelig, Vahid Sandoghdar

We perform wide-field microscopy to investigate the photobleaching of organic fluorophores embedded in the polymeric host PMMA. Our experimental arrangement facilitates the comparison between the ensemble and single molecule data. We characterize the photostability of dye molecules of various families by measuring the 'bleaching number', defined as the average number of photons a molecule emits until photobleaching occurs. In particular, we have analysed the dependence of the bleaching number on the presence of oxygen. Surprisingly, we find an improvement of photostability in the presence of oxygen for ionic dyes (DiI, TMR, Rh6G, Alexa 546), suggesting that oxygen quenches the photoactive triplet state, but it only indirectly contributes to photochemistry. In contrast, we observe that photobleaching of the aromatic hydrocarbon is strongly enhanced by oxygen.

Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna

Sergei Kuehn, Ulf Hakanson, Lavinia Rogobete, Vahid Sandoghdar

We investigate the coupling of a single molecule to a single spherical gold nanoparticle acting as a nanoantenna. Using scanning probe technology, we position the particle in front of the molecule with nanometer accuracy and measure a strong enhancement of more than 20 times in the fluorescence intensity simultaneous to a 20-fold shortening of the excited state lifetime. Comparisons with three-dimensional calculations guide us to decipher the contributions of the excitation enhancement, spontaneous emission modification, and quenching. Furthermore, we provide direct evidence for the role of the particle plasmon resonance in the molecular excitation and emission processes.

Modification of single molecule fluorescence by a scanning probe

S. Kuehn, Vahid Sandoghdar

We examine the optical near-field interaction between different types of scanning tips and single oriented fluorescent molecules. We demonstrate the influence of a tip on the excitation intensity as well as on the integrated fluorescence signal, the excited state lifetime, and the angular emission of single molecules. By using a standard model describing the radiation of an oscillating dipole close to a nanosphere or a flat interface, we interpret our observations and describe some central criteria for obtaining fluorescence enhancement or quenching.

Interferometric optical detection and tracking of very small gold nanoparticles at a water-glass interface

V. Jacobsen, P. Stoller, C. Brunner, V. Vogel, Vahid Sandoghdar

We use an interferometric detection scheme to directly detect single gold nanoparticles with a diameter as small as 5 nm in an aqueous environment. We demonstrate both confocal and wide-field detection of nanoparticles and study signal strength as a function of particle size. Furthermore, we demonstrate a detection speed up to 2 mu s. We also show that gold nanoparticles can be readily distinguished from background scatterers by exploiting the wavelength dependence of their plasmon resonances. Our studies pave the way for the application of this detection scheme for particle tracking in biological systems. (c) 2006 Optical Society of America.

Optical detection of very small nonfluorescent nanoparticles

Vahid Sandoghdar, Enrico Klotzsch, Volker Jacobsen, Alois Renn, Ulf Hakanson, Mario Agio, Ilja Gerhardt, Johannes D. Seelig, Gert Wrigge

We discuss an interferometric method for the optical detection of very small nonfluorescent nanoparticles. In particular, we show that single gold nanoparticles with a diameter as small as 5 nm can be detected. We discuss the potential of such tiny particles as optical labels for biological studies. Furthermore, we show that our interferometric method can be also used for the detection and tracking of unlabelled biological nano-entities such as viruses or microtubuli.

Optical detection of very small nonfluorescent nanoparticles

Vahid Sandoghdar, Enrico Klotzsch, Volker Jacobsen, Alois Renn, Ulf Hakanson, Mario Agio, Ilja Gerhardt, Johannes Seelig, Gert Wrigge

We discuss an interferometric method for the optical detection of very small nonfluorescent nanoparticles. In particular, we show that single gold nanoparticles with a diameter as small as 5 nm can be detected. We discuss the potential of such tiny particles as optical labels for biological studies. Furthermore, we show that our interferometric method can be also used for the detection and tracking of unlabelled biological nano-entities such as viruses or microtubuli.

Measurement of the complex dielectric constant of a single gold nanoparticle

Patrick Stoller, Volker Jacobsen, Vahid Sandoghdar

A differential interference contrast microscopy technique that employs a photonic crystal fiber as a white-light source is used to measure both the real and the imaginary part of the complex dielectric constant of single 10 and 15 nm gold nanoparticles over a wavelength range of 480 to 610 run. Noticeable deviations from bulk gold measurements are observed at short wavelengths and for individual particles even after taking into account finite-size surface damping effects. (c) 2006 Optical Society of America.

Spontaneous emission rates of dipoles in photonic crystal membranes

A. Femius Koenderink, Maria Kafesaki, Costas M. Soukoulis, Vahid Sandoghdar

We show theoretically that two-dimensional (2D) photonic crystals in semiconductor membranes strongly modify the radiative decay of dipole emitters. Three-dimensional finite-difference time-domain calculations show over 7 times inhibition and 15 times enhancement of the emission rate compared with vacuum for judiciously oriented and positioned dipoles. Emission rate modifications inside the membrane mimic the local mode density in a simple 2D model. The inhibition of emission saturates with crystal size around the source, with a 1/e size that scales as the inverse gap bandwidth. Owing to the vertically guided mode structure, inhibition occurs only near the slab center, but enhanced emission persists also outside the membrane. We find that emission changes can even be observed in experiments with ensembles of randomly oriented dipoles. (c) 2006 Optical Society of America.

Measuring the quantum efficiency of the optical emission of single radiating dipoles using a scanning mirror

B.C. Buchler, Thomas Kalkbrenner, C. Hettich, Vahid Sandoghdar

Using scanning probe techniques, we show the controlled manipulation of the radiation from single dipoles. In one experiment we study the modification of the fluorescence lifetime of a single molecular dipole in front of a movable silver mirror. A second experiment demonstrates the changing plasmon spectrum of a gold nanoparticle in front of a dielectric mirror. Comparison of our data with theoretical models allows determination of the quantum efficiency of each radiating dipole.

Spontaneous emission in the near field of two-dimensional photonic crystals

A. Femius Koenderink, Maria Kafesaki, Costas M. Soukoulis, Vahid Sandoghdar

We show theoretically that photonic crystal membranes cause large variations in the spontaneous emission rate of dipole emitters, not only inside but also in the near field above the membranes. Our three-dimensional finite-difference time-domain calculations reveal an inhibition of more than five times and an enhancement of more than ten times for the spontaneous emission rate of emitters with select dipole orientations and frequencies. Furthermore, we demonstrate theoretically the potential of a nanoscopic emitter attached to the end of a glass fiber tip as a local probe for mapping the large spatial variations of the photonic crystal local radiative density of states. This arrangement is promising for on-command modification of the coupling between an emitter and the photonic crystal in quantum optical experiments. (c) 2005 Optical Society of America.

Optical microscopy via spectral modifications of a nanoantenna

Thomas Kalkbrenner, Ulf Hakanson, A. Schadle, S. Burger, C. Henkel, Vahid Sandoghdar

The existing optical microscopes form an image by collecting photons emitted from an object. Here we report on the experimental realization of microscopy without the need for direct optical communication with the sample. To achieve this, we have scanned a single gold nanoparticle acting as a nanoantenna in the near field of a sample and have studied the modification of its intrinsic radiative properties by monitoring its plasmon spectrum.

Controlling the resonance of a photonic crystal microcavity by a near-field probe

A. Femius Koenderink, Maria Kafesaki, Ben C. Buchler, Vahid Sandoghdar

We demonstrate theoretically that the resonance frequencies of high-Q microcavities in two-dimensional photonic crystal membranes can be tuned over a wide range by introducing a subwavelength dielectric tip into the cavity mode. Three-dimensional finite-difference time-domain simulations show that by varying the lateral and vertical positions of the tip, it is possible to tune the resonator frequency without lowering the quality factor. Excellent agreement with a perturbative theory is obtained, showing that the tuning range is limited by the ratio of the cavity mode volume to the effective polarizability of the nanoperturber.

Optimization of prism coupling to high-Q modes in a microsphere resonator using a near-field probe

A. Mazzei, Stephan Götzinger, L. de S. Menezes, Vahid Sandoghdar, O. Benson

In this paper, we demonstrate a novel method for optimizing the in- and out-coupling of light confined in the fundamental high-Q whispering-gallery mode of a microsphere resonator via an external prism coupler. The technique relies on the use of a near-field probe to map the modes of the resonator and to obtain topographical information at the same time. We demonstrate the feasibility and efficiency of this technique by applying it to a sphere with a radius of 59 mu m. (c) 2005 Elsevier B.V. All rights reserved.

Near-field optics and control of photonic crystals

A. Femius Koenderink, R. Wuest, Ben C. Buchler, S. Richter, P. Strasser, Maria Kafesaki, A. Rogach, R.B. Wehrspohn, C.M. Soukoulis, et al.

We discuss recent progress and the exciting potential of scanning probe microscopy methods for the characterization and control of photonic crystals. We demonstrate that scanning near-field optical microscopy can be used to characterize the performance of photonic crystal device components on the sub-wavelength scale. In addition, we propose scanning probe techniques for realizing local, low-loss tuning of photonic crystal resonances, based on the frequency shifts that high-index nanoscopic probes can induce. Finally, we discuss prospects for on-demand spontaneous emission control. We demonstrate theoretically that photonic crystal membranes induce large variations in spontaneous emission rate over length scales of 50 nm that can be probed by single light sources, or nanoscopic ensembles of light sources attached to the end of a scanning probe. (c) 2005 Elsevier B.V. All rights reserved.

A "standing-wave meter" to measure dispersion and loss of photonic-crystal waveguides

R. Wuest, D. Erni, P. Strasser, F. Robin, H. Jackel, B.C. Buchler, A.F. Koenderink, Vahid Sandoghdar, R. Harbers

We demonstrate a "standing-wave meter" for measuring dispersion and loss along the length of a planar InP-based photonic-crystal waveguide. Light from a tunable cw laser was coupled into a single line-defect waveguide that terminated inside the crystal structure to form a retroreflector. This structure created a standing wave which was imaged using a scanning near-field optical microscope. By measuring the intensity distribution of the standing wave for a range of optical frequencies, waveguide dispersion and loss were measured with high accuracy. Comparisons of the measurement results with three-dimensional numerical simulations reveal that material dispersion effects as small as 0.8% affect the band structure measurably. (c) 2005 American Institute of Physics.

Tomographic plasmon spectroscopy of a single gold nanoparticle

Thomas Kalkbrenner, Ulf Hakanson, Vahid Sandoghdar

We demonstrate a tomographic method for determining the degree of ellipticity and the orientation of a gold nanoparticle. To do this, we attach a single nanoparticle to the end of a sharp glass fiber tip and record its plasmon spectra for different incident polarizations and angles of incidence. Our measurements allow us to identify the plasmon spectra along the three main axes of the particle, therefore resolving its "internal" spectral inhomogeneity. Knowledge of the plasmon resonances and the orientation of a metallic nanoparticle is an important asset for controlled and quantitative studies of its interaction with a single molecule.

Aligned terrylene molecules in a spin-coated ultrathin crystalline film of p-terphenyl

R.J. Pfab, J. Zimmermann, C. Hettich, Ilja Gerhardt, Alois Renn, Vahid Sandoghdar

We report on the use of a simple spin casting procedure to fabricate very thin crystalline films of P-terphenyl doped with fluorescent terrylene molecules. By performing single molecule studies, we show that the guest molecules are oriented normal to the plane of the film. We find that despite the very low thickness of the p-terphenyl matrix. as thin as only 20 molecular layers, about half of the embedded emitters withstand photobleaching for illumination times of at least a day. (C) 2004 Elsevier B.V. All rights reserved.

Near-field optical investigations of photonic crystal microresonators

B.C. Buchler, P. Kramper, M. Kafesaki, C.M. Soukoulis, Vahid Sandoghdar

We present an overview of our work on the application of scanning near-field optical microscopy (SNOM) to photonic crystal structures. Our results show that SNOM can be used to map the subwavelength confinement of light to a point-defect in a 2D photonic crystal microresonator. Comparison with numerical modelling shows that SNOM is able to resolve patterns in the intensity distribution that are due to the slight non-uniformity in the crystal structure. We also discuss the future possibilities for applications of different modes of SNOM to photonic crystal devices.

Confocal microscopy and spectroscopy of nanocrystals on a high-Q microsphere resonator

Stephan Götzinger, L. de S. Menezes, O. Benson, D.V. Talapin, N. Gaponik, H. Weller, A.L. Rogach, Vahid Sandoghdar

We report on experiments where we used a home-made confocal microscope to excite single nanocrystals on a high-Q microsphere resonator. In that way spectra of an individual quantum emitter could be recorded. The Q factor of the microspheres coated with nanocrystals was still up to 10(9). We also demonstrate the use of a prism coupler as a well-defined output port to collect the fluorescence of an ensemble of nanocrystals coupled to whispering-gallery modes.

Detection and spectroscopy of gold nanoparticles using supercontinuum white light confocal microscopy

Klas Lindfors, Thomas Kalkbrenner, Patrick Stoller, Vahid Sandoghdar

We combine confocal microscopy using supercontinuum laser illumination and an interferometric detection technique to identify single nanoparticles of diameter below 10 nm. Spectral analysis of the signal allows us to record the plasmon resonance of a single nanoparticle. Our results hold great promise for fundamental studies of the optical properties of single metal clusters and for their use in biophysical applications.

Near-field visualization of light confinement in a photonic crystal microresonator

Patrcik Kramper, Maria Kafesaki, Costas M. Soukoulis, Albert Birner, Frank Muller, Ulrich Goesele, Ralf B. Wehrspohn, Jürgen Mlynek, Vahid Sandoghdar

By using scanning near-field optical microscopy, we directly map the subwavelength confinement of light around a point defect in a two-dimensional photonic crystal microresonator. Comparison of our results with the outcome of three-dimensional finite-difference time domain calculations allows us to identify small imperfections in the structure that result in the spatial modification of the intensity distribution. (C) 2004 Optical Society of America

Highly directional emission from photonic crystal waveguides of subwavelength width

P. Kramper, M. Agio, C.M. Soukoulis, A. Birner, F. Muller, R.B. Wehrspohn, U. Goesele, Vahid Sandoghdar

Recently it has been shown that it is possible to achieve directional emission out of a subwavelength aperture in a periodically corrugated metallic thin film. We report on theoretical and experimental studies of a related phenomenon concerning light emitted from photonic crystal waveguides that are less than a wavelength wide. We find that the termination of the photonic crystal end facets and an appropriate choice of the wavelength are instrumental in achieving very low numerical apertures. Our results hold promise for the combination of photonic crystal waveguides with conventional optical systems such as fibers, waveguides, and freely propagating light beams.

Subwavelength emitters in the near-infrared based on mercury telluride nanocrystals

P. Olk, B.C. Buchler, Vahid Sandoghdar, N. Gaponik, A. Eychmuller, A.L. Rogach

The optical properties of near-infrared-emitting mercury telluride (HgTe) nanocrystals are investigated using fluorescence confocal microscopy. The HgTe quantum dots were coated with a "layer-by-layer" process on latex spheres with diameters down to 150 nm and number of layers varying from 1 to 3. The individual spheres act as bright subwavelength light sources. Measurements of the fluorescence decay show that the lifetime varies with the number of nanocrystal layers. (C) 2004 American Institute of Physics.

Spontaneous emission in nanoscopic dielectric particles

Lavinia Rogobete, Hannes Schniepp, Vahid Sandoghdar, Carsten Henkel

We report on theoretical studies of the inhibition of the spontaneous emission process in subwavelength dielectric media. We discuss the modification of the spontaneous emission rate as a function of the size and shape of the medium as well as the position of the emitter in it. (C) 2003 Optical Society of America.

Spontaneous emission of europium ions embedded in dielectric nanospheres

Hannes Schniepp, Vahid Sandoghdar

We measure fluorescence lifetimes of emitters embedded in isolated single dielectric nanospheres. By varying the diameters of the spheres from 100 nm to 2 mum and by modifying their dielectric surrounding, we demonstrate a systematic change of paradigm in the spontaneous emission rate, as we cross the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering. Our data show inhibition of the spontaneous emission up to 3 times and are in excellent agreement with the results of analytical calculations.

Nanometer resolution and coherent optical dipole coupling of two individual molecules

C. Hettich, C. Schmitt, J. Zitzmann, S. Kuhn, Ilja Gerhardt, Vahid Sandoghdar

By performing cryogenic laser spectroscopy under scanning probe electrode that induces local electric field, we have resolved two individual fluorescent molecules separated by 12 nanometers in an organic crystal. The two molecules undergo strong coherent dipole-dipole coupling that produces entangled sub- and superradiant states. Under intense laser illumination, both molecules are excited via two-photon transition, and the fluorescence from this doubly excited system displays photon bunching. Our experimental scheme can be used to optically resolve molecules at the nanometer scale and to manipulate the degree of entanglement among them.

Influence of a sharp fiber tip on high-Q modes of a microsphere resonator

Stephan Götzinger, O. Benson, Vahid Sandoghdar

We investigate the degradation of the Q factor of a fundamental whispering-gallery mode of a microsphere resonator when a fiber tip is placed in the evanescent field of the mode. With a tip diameter of 80 nm it is possible to maintain a Q factor exceeding 10(8), even when the tip is as close as 10 nm to the sphere surface. This result demonstrates the possibility of using such a tip as a "nanotool" to actively place a single nanoparticle in a single high-Q mode with great precision to achieve well-controlled coupling. (C) 2002 Optical Society of America.

Apertureless near-field optical microscopy via local second-harmonic generation

Anatoly V. Zayats, Vahid Sandoghdar

We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip-surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.

Results and thoughts on optical microscopy using a single-molecule probe

Vahid Sandoghdar, J. Michaelis, C. Hettich, C. Schmitt, J. Zitzmann, S. Kuhn

We discuss our recent results as well as planned experiments for obtaining molecular resolution in optical microscopy. By combining single molecule spectroscopy and scanning probe microscopy we have demonstrated optical microscopy with a single-molecule light source. We also sketch a method for the use of an inhomogeneous static electric field for determining the position of a molecule in all three dimensions.

Beating the diffraction limit

Vahid Sandoghdar

The optical microscope is probably the only scientific instrument that children are familiar with, and possibly the only one that can be bought in toy shops. At school, most of us looked into such microscopes with great expectations, hoping to see unimaginable details of a leaf or dead fly. Often we were disappointed that we could not see more. Where were all those atoms and molecules that everything is supposed to be made of? If we complained to the teacher, we were usually told that one cannot see things that are too small in an optical microscope. Until recently the teacher was right, but not any more.

Diamond colour centres as a nanoscopic light source for scanning near-field optical microscopy

S. Kuhn, C. Hettich, C. Schmitt, J-PH. Poizat, Vahid Sandoghdar

Recently it was shown that a single molecule at cryogenic temperatures could be used as a local light source for illumination of a sample in the near field. Conventional light-emitting systems such as dye molecules and semiconductor quantum dots could also be used for this purpose, but they suffer from lack of photostability. However, colour centres in diamond have been found to be remarkably stable against bleaching and blinking effects. Here we present the fist SNOM images taken with nanoscopic diamond crystals as a light source.

Direct spectroscopy of a deep two-dimensional photonic crystal microresonator

P. Kramper, A. Birner, Mario Agio, C.M. Soukoulis, F. Muller, U. Gosele, Jürgen Mlynek, Vahid Sandoghdar

Photonic crystals based on macroporous silicon with fundamental band gaps in the middle infrared region 3.4-5.8 mum were fabricated. Scanning probe optical microscopy and laser spectroscopy were combined to examine a deep two-dimensional photonic crystal microresonator based on a single point defect. Two sharp resonances were recorded in the band gap, in excellent agreement with the results of numerical simulations. Such a microresonator with high-quality factors and a subwavelength mode extension could be used for a range of applications including integrated optical gas sensors.

Towards controlled coupling between a high-Q whispering-gallery mode and a single nanoparticle

Stephan Götzinger, O. Benson, Vahid Sandoghdar

We discuss our recent experiments that aim at the realization of coupling between a nano-emitter that is placed at the extremity of a sharp glass-fiber tip and a high-Q whispering-gallery mode. We quantify the influence of the tip using different probes and modes of a microsphere with different quality factors and mode extensions. Our measurements show that a micron-sized tip results in a substantial perturbation of the modes. On the contrary, by using a tip of diameter about 100 nm it should be possible to couple a nanoparticle to the most-confined modes of a microsphere without spoiling quality factors even as high as 10(8).

Mapping and manipulating whispering gallery modes of a microsphere resonator with a near-field probe

Stephan Götzinger, S. Demmerer, O. Benson, Vahid Sandoghdar

We report high spatial resolution mapping of high-Q whispering gallery modes in microsphere resonators with a near-field probe. We present experimental results on the effect of Q-factor degradation when the probe interacts with the evanescent field and discuss future applications of our experimental set-up for realization of novel nanolasers and nano light-emitting-diodes.

A single gold particle as a probe for apertureless scanning near-field optical microscopy

Thomas Kalkbrenner, M. Ramstein, Jürgen Mlynek, Vahid Sandoghdar

We report on the fabrication, characterization and application of a probe consisting of a single gold nanoparticle for apertureless scanning near-field optical microscopy. Particles with diameters of 100 nm have been successfully and reproducibly mounted at the end of sharp glass fibre tips. We present the first optical images taken with such a probe. We have also recorded plasmon resonances of gold particles and discuss schemes for exploiting the wavelength dependence of their scattering cross-section for a novel form of apertureless scanning near-field optical microscopy.

A model system for two-dimensional and three-dimensional photonic crystals: macroporous silicon

J. Schilling, R.B. Wehrspohn, A. Birner, F. Muller, R. Hillebrand, U. Goesele, S.W. Leonard, J.P. Mondia, F. Genereux, et al.

A review of the optical properties of two-dimensional and three-dimensional photonic crystals based on macroporous silicon is given. As macroporous silicon provides structures with aspect ratios exceeding 100, it can be considered to be an ideal two-dimensional photonic crystal. Most of the features of the photonic dispersion relation have been experimentally determined and were compared to theoretical calculations. This includes transmission and reflection of finite and bulk photonic crystals and their variation with the pore radius to determine the gap map. All measurements have been carried out for both polarizations separately since they decouple in two-dimensional photonic crystals. Moreover, by inhibiting the growth of selected pores, point and line defects were realized and the corresponding high-Q microcavity resonances as well as waveguiding properties were studied via transmission. The tunability of the bandgap was demonstrated by changing the refractive index inside the pores caused by an infiltrated liquid crystal undergoing a temperature-induced phase transition. Finally different realizations of three-dimensional photonic crystals using macroporous silicon are discussed. In all cases an excellent agreement between experimental results and theory is observed.

High-contrast topography-free sample for near-field optical microscopy

Thomas Kalkbrenner, M Graf, C Durkan, Jürgen Mlynek, Vahid Sandoghdar

The issue of topography artifacts has proven to play a very important role in interpreting images recorded in scanning near-field optical microscopy. We report on the fabrication and characterization of samples with essentially no topographic features while possessing very high optical contrast on the nanometric lateral scale. These samples open the door to routine and uncontroversial examinations of the resolution obtained in a scanning near-field optical microscope. (C) 2000 American Institute of Physics. [S0003-6951(00)04409-0].

Apertureless scanning near-field second-harmonic microscopy

Anatoly V. Zayats, Vahid Sandoghdar

We propose a new type of apertureless scanning near-field optical microscope based on detection of the second-harmonic signal induced by or generated at a probe tip. We discuss the image formation in this technique and present numerical studies for different experimental circumstances. In two extreme cases this method is shown to be the nonlinear analogue of apertureless scanning near-field optical microscopy based on local nanoscopic fluorescence or scattering. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Second-harmonic generation from individual surface defects under local excitation

Anantoly V. Zayats, Thomas Kalkbrenner, Vahid Sandoghdar, Jürgen Mlynek

Enhancement of optical second-harmonic generation (SHG) at individual defects on metal surface has been studied. SHG has been excited locally at chosen defects using a near-field optical microscope with an uncoated fiber tip. SH intensity enhancement up to ten times has been observed at the apex of micron size defects on a gold surface while the average enhancement is of about 1.2 times. Observed SHG enhancement has been described by lightning rod effect. Specific features of SHG enhancement due to local excitation are briefly discussed.

Transmission of a microcavity structure in a two-dimensional photonic crystal based on macroporous silicon

A. Birner, A.-P. Li, F. Muller, U. Gosele, P. Kramper, Vahid Sandoghdar, Jürgen Mlynek, K. Busch, V. Lehmann

Photonic crystals consist of regularly arranged dielectric scatterers of dimensions on a wavelength scale, exhibiting band gaps for photons, analogous to the case of electrons in semiconductors. Using electrochemical pore formation in n-type silicon, we fabricated photonic crystals consisting of air cylinders in silicon. The starting positions of the pores were photolithographically pre-defined to form a hexagonal lattice of a = 1.58 mum. The photonic crystal was microstructured to make the photonic lattice accessible for optical characterization. Samples with different filling factors were fabricated to verify the gap map of electric and magnetic modes using Fourier-transform infrared (IR) spectroscopy. The complete band gap could be tuned from 3.3 to 4.3 mum wavelength. We were able to embed defects such as waveguide structures or microcavities by omitting certain pores. We carried out transmission measurements using a tunable mid-IR optical parametric oscillator. The resonance is compared with theoretical expectations. (C) 2001 Elsevier Science Ltd. All rights reserved.

Optical microscopy using a single-molecule light source

J. Michaelis, C. Hettich, Jürgen Mlynek, Vahid Sandoghdar

Rapid progress in science on nanoscopic scales has promoted increasing interest in techniques of ultrahigh-resolution optical microscopy. The diffraction limit can be surpassed by illuminating an object in the near field through a sub-wavelength aperture at the end of a sharp metallic probe(1,2). Proposed modifications(3,4) of this technique involve replacing the physical aperture by a nanoscopic active light source. Advances in the spatial(5) and spectral(6) detection of individual fluorescent molecules, using near-field and far-field methods(7), suggest the possibility of using a single molecule(8,9) as the illumination source. Here we present optical images taken with a single molecule as a point-like source of illumination, by combining fluorescence excitation spectroscopy(10) with shear-force microscopy(11). Our single-molecule probe has potential for achieving molecular resolution in optical microscopy; it should also facilitate controlled studies of nanometre-scale phenomena (such as resonant energy transfer) with improved lateral and axial spatial resolution.

Multifunctional AFM/SNOM cantilever probes: Fabrication and measurements

M. Stopka, D. Drews, K. Mayr, M. Lacher, W. Ehrfeld, T.. Kalkbrenner, M Graf, Vahid Sandoghdar, Jürgen Mlynek

The microfabrication process for cantilever probes for combined atomic force (AFM) and scanning near-field optical microscopy (SNOM) is described. The probes feature an aperture tip with a Si3N4 core for SNOM operation as well as an integrated optical waveguide for illumination of the tip. First measurements have been performed using a home-made AFM/SNOM setup operating in tapping mode with optical beam deflection. A special test sample containing a pattern of gold nanostructures embedded in a transparent polymer matrix provides low topography but high optical contrast. An optical resolution of about 100 nm has been demonstrated by examining the contrast in transmitted intensity at the metal/polymer border.

A single molecule as a probe of optical intensity distribution

J. Michaelis, C. Hettich, V. Zayats, B. Eiermann, Jürgen Mlynek, Vahid Sandoghdar

Single terrylene molecules embedded in microscopic p-terphenyl crystals are identified with the technique of fluorescence excitation spectroscopy. By use of the architecture of a scanning-probe microscope at T = 1.4 K, a single molecule is scanned through an excitation laser beam while the fluorescence signal is recorded. In this manner we have mapped the intensity distribution in a one-dimensional optical standing wave, demonstrating the potential of a single molecule as a nanometric probe. We discuss future experiments aimed at combining the high spatial and spectral sensitivity of a single molecule. (C) 1999 Optical Society of America.

Prospects of apertureless SNOM with active probes

Vahid Sandoghdar, Jürgen Mlynek

We discuss the theoretical and experimental considerations of performing scanning near-field optical microscopy using subwavelength active media. In particular, we describe our efforts to realize such experiments using both finite-size nanoscopic probes and probes consisting of a single molecule as the source. In conclusion, we briefly address potential applications of these probes to other areas of science.

A novel fabrication method for fluorescence-based apertureless scanning near-field optical microscope probes

P. Kramper, A. Jebens, T. Muller, Jürgen Mlynek, Vahid Sandoghdar

We report a novel method for the fabrication of probes with localized sub-wavelength fluorescing media at their extremities. We present our first results and discuss future plans to extend this technique to the systematic fabrication of fluorescent probes for apertureless scanning near-field optical microscopy.

Single-molecule spectroscopy near structured dielectrics

Carsten Henkel, Vahid Sandoghdar

We present an analytical approach to the calculation of the linewidth and lineshift of an atom or molecule in the near field of a structured dielectric surface. For soft surface corrugations with amplitude lambda/50, we find variations of the linewidth in the ten percent region. More strikingly, the shift of the molecular resonance can reach several natural linewidths. We demonstrate that the lateral resolution is of the order of the molecule-surface distance. We give a semiquantitative explanation of the outcome of our calculations that is based on simple intuitive models. (C) 1998 Elsevier Science B.V. All rights reserved.

Lithography using nano-lens arrays made of light

Vahid Sandoghdar, U. Drodofsky, Th. Schulze, B. Brezger, M. Drewsen, T. Pfau, Jürgen Mlynek

We discuss the fabrication of various one and two-dimensional chromium nanostructures. A chromium atomic beam effusing out of a thermal oven is first transversally laser cooled. This well-collimated beam is then structured and focused by the optical dipole force in a periodic light field. By introducing a substrate at the focus of the atomic beam we are able to capture this pattern directly. We discuss a few applications and challenges of this young lithography technique.

Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?

Vahid Sandoghdar, S. Wegscheider, G. Krausch, Jürgen Mlynek

We have investigated the optical resolution of a scanning near-field optical microscope in reflection collection mode using an uncoated fiber tip. We demonstrate that the apparent resolution in the optical signal (better than 70 nm) is a topography-induced effect. We believe that the purely optical resolution is only of the order of lambda/2 and diffraction limited. (C) 1997 American Institute of Physics.

Very low threshold whispering-gallery-mode microsphere laser

Vahid Sandoghdar, F. Treussart, J. Hare, V. LefevreSeguin, J.M. Raimond, S. Haroche

We report on the realization of a whispering-gallery-mode laser based on neodymium-doped silica microspheres. Absorbed pump powers at threshold are as low as 200 nW. The linear variation of the threshold with the loss factor of the cavity mode has also been observed. We discuss the potential of this system as a permanent microlaser operating with a few active ions at liquid-helium temperature.

Characterizing whispering-gallery modes in microspheres by direct observation of the optical standing-wave pattern in the near field

J.C. Knight, N. Dubreuil, Vahid Sandoghdar, J. Hare, V. LefevreSeguin, J.M. Raimond, S. Haroche

We demonstrated the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator. The periodicity of the observed standing wave allows us to estimate accurately the radial mode number of the whispering-gallery mode resonance that is being excited. We find that the use of a fiber half-coupler to excite these resonances in fused-silica microspheres results in only the lowest radial mode numbers' being strongly excited, as predicted. (C) 1996 Optical Society of America

Spectroscopy of atoms confined to the single node of a standing wave in a parallel-plate cavity

Vahid Sandoghdar, C.I. Sukenik, S. Haroche, E.A. Hinds

have performed spectroscopy on sodium atoms that are optically channeled in the single node of a laser standing wave set up across a parallel-plate cavity. Using this technique we have extended our previous measurement of the Lennard-Jones van der Waals energy-level shift [Sandoghdar et al., Phys. Rev. Lett. 68, 3432 (1992)] down to a cavity width of similar to 500 nm. We discuss the applications of this technique to the precise measurement of atom-surface distances.

Mapping whispering-gallery modes in microspheres with a near-field probe

J.C. Knight, N. Dubreuil, Vahid Sandoghdar, J. Hare, V. Lefèvre-Seguin, J.M. Raimond, S. Haroche

We demonstrate the use of a near-field probe to map the angular dependence of high-and whispering-gallery modes in fused-silica microspheres. The mapping is performed by placing a micrometer-sized tip formed on the end of a monomode fiber into the evanescent field at the microsphere surface, causing Light to be coupled from the microsphere resonance into the fiber guided mode. The light output of the fiber is then measured while the tip is moved to different points on the microsphere surface. We have used this method to investigate the lifting of spherical degeneracy in the system.

Splitting of high-Q Mie modes induced by light backscattering in silica microspheres

D.S. Weiss, Vahid Sandoghdar, J. Hare, V. Lefèvre-Seguin, J.M. Raimond, S. Haroche

We have observed that very high-and Mie resonances in silica microspheres are split into doublets. This splitting is attributed to internal backscattering that couples the two degenerate whispering-gallery modes propagating in opposite directions along the sphere equator. We have studied this doublet structure by high-resolution spectroscopy. Time-decay measurements have also been performed and show a beat note corresponding to the coupling rate between the clockwise and counterclockwise modes. A simple model of coupled oscillators describes our data well, and the backscattering efficiency that we measure is consistent with what is observed in optical fibers. (C) 1995 Optical Society of America

Eroded monomode optical fiber for whispering-gallery mode excitation in fused-silica microspheres

N. Dubreuil, J.C. Knight, D.K. Leventhal, Vahid Sandoghdar, J. Hare, V. Lefèvre

We demonstrate the efficient excitation of high-Q whispering-gallery modes in near-spherical fused-silica microparticles in the size range 60-450 mu m by the use of an eroded monomode optical fiber. When the sphere is placed in the evanescent field of the guided fiber mode, light is resonantly coupled from the fiber into the microparticle. We report a broadening of resonance modes and a shift of the resonance central frequency as the coupling strength is increased by reduction of the gap between the sphere and the fiber.

Quantized atom-field force at the surface of a microsphere

F. Treussart, J. Hare, L. Collot, V. Lefevre, D.S. Weiss, Vahid Sandoghdar, J.M. Raimond, S. Haroche

The dipole force experienced by an atom in a nonresonant spatially inhomogeneous light field is quantized by the discrete nature of the photon. We propose to detect this quantization by studying the scattering of slow atoms that pass in the evanescent field of a microsphere whispering gallery mode. This constitutes an inverse Stern-Gerlach experiment in which the atomic deflection is correlated to the state of the scattering field.

Measurement of the Casimir-Polder force

C.I. Sukenik, M.G. Boshier, D. Cho, Vahid Sandoghdar, E.A. Hinds

We have studied the deflection of ground-state sodium atoms passing through a micron-sized parallel-plate cavity by measuring the intensity of a sodium atomic beam transmitted through the cavity as a function of cavity plate separation. This experiment provides clear evidence for the existence of the Casimir-Polder force, which is due to modification of the ground-state Lamb shift in the confined space of a cavity. Our results confirm the magnitude of the force and the distance dependence predicted by quantum electrodynamics.

Direct measurement of the van der Waals interaction between an atom and its images in a micron-sized cavity

Vahid Sandoghdar, C.I. Sukenik, E.A. Hinds, Serge Haroche

We have measured by laser spectroscopy the energy of interaction between a sodium atom and its images in the walls of a micron-sized cavity. This cavity-QED study is the first direct quantitative test of the Lennard-Jones van der Waals interaction as a function of controlled atom-surface separation and mean-square electric dipole moment.

Cavity QED level shifts of simple atoms

E. A. Hinds, Vahid Sandoghdar

We have made a quantitative study of quantum electrodynamic corrections to the energy of some simple atoms near a metallic surface. (i) The two-level atom provides a basic framework within which we discuss the van der Waals, Casimir, and resonant radiative level shifts. In this discussion we show that the level shifts of an excited atom are substantially different from those of a classical dipole antenna. (ii) We calculate the possible cavity QED corrections to the n = 2 and n = 1 hydrogen Lamb-shift measurements and obtain results that disagree with the published literature. (iii) We find a general expression for the shifts of the lowest S and P levels of alkali-metal atoms near a conducting surface. These might be studied in the laboratory as a test of cavity QED.

External-cavity frequency-stabilization of visible and infrared semiconductor lasers for high resolution spectroscopy

M.G. Boshier, D. Berkeland, E.A. Hinds, Vahid Sandoghdar

We have shown that external cavity stabilization can be a straightforward and powerful technique for converting diode lasers of poor spectral quality into useful tools for high precision laser spectroscopy. The method requires an anti-reflection coating on one facet of the diode laser but, contrary to popular belief, this is not difficult and does not require any specialized equipment. We describe a coating procedure which can be used with standard commercial diode lasers. We have demonstrated the external cavity method using diodes at a variety of visible and infrared wavelengths. Details are given for a visible laser centered at 670 nm.