Prof. Stephan Götzinger

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.

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<br>(DBT) molecules doped in anthracene crystals of nanoscopic height fabricated by electrohydrodynamic dripping.<br>Using high-resolution fluorescence excitation spectroscopy, we show that zero-phonon lines of single molecules<br>in printed nanocrystals are nearly as narrow as the Fourier-limited transitions observed for the same guest-host<br>system in the bulk. Moreover, the spectral instabilities are comparable to or less than one linewidth. By<br>recording super-resolution images of DBT molecules and varying the polarization of the excitation beam, we<br>determine the dimensions of the printed crystals and the orientation of the crystals’ axes. Electrohydrodynamic<br>printing of organic nano and microcrystals paves the way for a series of applications, where controlled positioning<br>of quantum emitters with narrow optical transitions is desirable.

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

Organic Molecules as Origin of Visible-Range Single Photon Emission from Hexagonal Boron Nitride and Mica

Michael Neumann, Xu Wei, Luis Morales-Inostroza, Seunghyun Song, Sung-Gyu Lee, Kenji Watanabe, Takashi Taniguchi, Stephan Götzinger, Young Hee Lee

The discovery of room-temperature single-photon emitters (SPEs) hosted by two-dimensional hexagonal boron nitride (2D hBN) has sparked intense research interest. Although emitters in the vicinity of 2 eV have been studied extensively, their microscopic identity has remained elusive. The discussion of this class of SPEs has centered on point defects in the hBN crystal lattice, but none of the candidate defect structures have been able to capture the great heterogeneity in emitter properties that is observed experimentally. Employing a widely used sample preparation protocol but disentangling several confounding factors, we demonstrate conclusively that heterogeneous single-photon emission at ∼2 eV associated with hBN originates from organic molecules, presumably aromatic fluorophores. The appearance of those SPEs depends critically on the presence of organic processing residues during sample preparation, and emitters formed during heat treatment are not located within the hBN crystal as previously thought, but at the hBN/substrate interface. We further demonstrate that the same class of SPEs can be observed in a different 2D insulator, fluorophlogopite mica.

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.

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.<br>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<br>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<br>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.

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 high resolution<br>in solid matrices. We present new spectroscopic measurements on single polycyclic aromatic molecules of dibenzoter-<br>rylene embedded in an organic crystal made of para-dichlorobenzene. To do this, we use narrow-band continuous-wave<br>lasers and combine spectroscopy methods based on fluorescence excitation and stimulated emission depletion (STED)<br>to select individual vibronic transitions at a resolution of ∼30 MHz dictated by the linewidth of the electronic ex-<br>cited state. In this fashion, we identify several exceptionally narrow vibronic levels in the electronic ground state with<br>linewidths down to values around 2 GHz. Additionally, we sample the distribution of vibronic wavenumbers, relax-<br>ation rates, and Franck-Condon factors, both in the electronic ground and excited states for a handful of individual<br>molecules. We discuss various noteworthy experimental findings and compare them with the outcome of DFT cal-<br>culations. The highly detailed vibronic spectra obtained in our work pave the way for studying the nanoscopic local<br>environment of single molecules. The approach also provides an improved understanding of the vibrational relaxation<br>mechanisms in the electronic ground state, which may help to create long-lived vibrational states for applications in<br>quantum technology.

Single photon sources for quantum radiometry: a brief review about the current state‑of‑the‑art

Stefan Kück, Marco López, Helmuth Hofer, Hristina Georgieva, Justus Christinck, Beatrice Rodiek, Geiland Porrovecchio, Marek Smid, Stephan Götzinger, et al.

Single-photon sources have a variety of applications. One of these is quantum radiometry, which is reported on in this<br>paper in the form of an overview, specifically of the current state of the art in the application of deterministic single photon<br>sources to the calibration of single photon detectors. To optimize single-photon sources for this purpose, extensive research<br>is currently carried out at the European National Metrology Institutes (NMIs), in collaboration with partners from universi-<br>ties. Single-photon sources of different types are currently under investigation, including sources based on defect centres in<br>(nano-)diamonds, on molecules and on semiconductor quantum dots. We will present, summarise, and compare the current<br>results obtained at European NMIs for single-photon sources in terms of photon flux, single-photon purity, and spectral<br>power distribution as well as the results of single-photon detector calibrations carried out with this type of light sources.

Comparison of back focal plane imaging of nitrogen vacancy centers in nanodiamond and core-shell CdSe/CdS quantum dots

Justus Christinck, Beatrice Rodiek, Marco Lopez , Hristina Georgieva, Stephan Götzinger, Stefan Kück

We report on the characterization of the angular-dependent emission of two different <br>single-photon emitters based on nitrogen-vacancy centers in nanodiamond and on core-shell CdSe/CdS quantum dot nanoparticles. The emitters were characterized in a confocal microscope <br>setup by spectroscopy and Hanbury-Brown and Twiss interferometry. The angular-dependent emission is measured using a back focal plane imaging technique. A theoretical model of the angular emission patterns of the 2D dipoles of the emitters is developed to determine their orientation. Experiment and model agree well with each other.

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

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.

Grain Dependent Growth of Bright Quantum Emitters in Hexagonal Boron Nitride

Noah Mendelson, Luis Morales-Inostroza, Chi Li, Ritika Ritika, Minh Anh Phan Nguyen, Jacqueline Loyola-Echeverria, Sejeong Kim, Stephan Götzinger, Milos Toth, et al.

Point defects in hexagonal boron nitride have emerged as a promising quantum light source due to their bright and photostable room temperature emission. In this work, the incorporation of quantum emitters during chemical vapor deposition growth on a nickel substrate is studied. Combining a range of characterization techniques, it is demonstrated that the incorporation of quantum emitters is limited to (001) oriented nickel grains. Such emitters display improved emission properties in terms of brightness and stability. These emitters are further utilized and integrated with a compact optical antenna enhancing light collection from the sources. The hybrid device yields average saturation count rates of ≈2.9 × 106 cps and an average photon purity of g(2)(0) ≈ 0.1. The results advance the understanding of single photon emitter incorporation during chemical vapor deposition growth and demonstrate a key step towards compact devices for achieving maximum collection efficiency.

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

Truncated Metallo-Dielectric Omnidirectional Reflector: Collecting Single Photons in the Fundamental Gaussian Mode with 95% Efficiency

Wancong Li, Luis Morales-Inostroza, Weiwang Xu, Pu Zhang, Stephan Götzinger, Xue-Wen Chen

We propose a novel antenna structure that funnelssingle photons from a single emitter with unprecedented efficiencyinto a low-divergence fundamental Gaussian mode. Our devicerelies on the concept of creating an omnidirectional photonicbandgap to inhibit unwanted large-angle emission and to enhancesmall-angle defect-guided-mode emission. The new photoncollection strategy is intuitively illustrated, rigorously verified,and optimized by implementing an efficient, body-of-revolution,finite-difference, time-domain method for in-plane dipole emitters.We investigate a few antenna designs to cover various boundaryconditions posed by fabrication processes or material restrictions and theoretically demonstrate that collection efficiencies into thefundamental Gaussian mode exceeding 95% are achievable. Our antennas are broadband, insensitive to fabrication imperfections andcompatible with a variety of solid-state emitters such as organic molecules, quantum dots, and defect centers in diamond.Unidirectional and low-divergence Gaussian-mode emission from a single emitter may enable the realization of a variety of photonicquantum computer architectures as well as highly efficient light−matter interfaces.

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.

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.

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.

Controlled generation of intrinsic near-infrared color centers in 4H-SiC via proton irradiation and annealing

M. Ruehl, C. Ott, Stephan Götzinger, M. Krieger, H.B. Weber

We report on the generation and annihilation of color centers in 4H silicon carbide (SiC) by proton irradiation and subsequent annealing. Using low-temperature photoluminescence (PL), we study the transformation of PL spectra for different proton doses and annealing temperatures. Among well reported defect signatures, we observe omnipresent but not yet identified PL signatures consisting of three sharp and temperature stable lines (denoted TS1,2,3) at 768.8 nm, 812.0 nm, and 813.3 nm. These lines show a strong correlation throughout all measurement parameters, suggesting that they belong to the same microscopic defect. Further, a clear dependence of the TS1,2,3 line intensities on the initial implantation dose is observed after annealing, indicating that the underlying defect is related to implantation induced intrinsic defects. The overall data suggest a sequential defect transformation: proton irradiation initially generates isolated silicon vacancies which are transformed into antisite vacancy complexes which are, in turn, transformed into presumably intrinsic-related defects, showing up as TS1,2,3 PL lines. We present recipes for the controlled generation of these color centers. Published by AIP Publishing.

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.

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

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

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.

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).

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%.

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.

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.

Synthesis of a Covalent Monolayer Sheet by Photochemical Anthracene Dimerization at the Air/Water Interface and its Mechanical Characterization by AFM Indentation

Payam Payamyar, Khaled Kaja, Carlos Ruiz-Vargas, Andreas Stemmer, Daniel J. Murray, Carey J. Johnson, Benjamin T. King, Florian Schiffmann, Joost VandeVondele, et al.

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.

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

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.

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).

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

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.

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]

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).