Dr. Tobias Utikal

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.

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.

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.

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.

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

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.

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

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.

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.

Spectroscopy of Graphene at the Saddle Point

D. Wolf, D.-H. Chae, T. Utikal, P. Herlinger, J. Smet, H. Giessen, M. Lippitz

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.

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).

Nonlinear Optics with Single Molecules

B. Gmeiner, A. Maser, T. Utikal, S. Goetzinger, V. Sandoghdar

We report on four-wave mixing in a single organic molecule placed at the tight focus of two near resonant laser beams. By directly monitoring the intensity of a weak probe beam after the interaction with the molecule, we observe a rich set of resonance profiles in excellent agreement with theory. (C) 2014 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.

Nano-Quantenoptik

Tobias Utikal, Emanuel Eichhammer, Benjamin Gmeiner, Andreas Maser, Daqing Wang, Pierre Türschmann, Hrishikesh Kelkar, Nir Rotenberg, Stephan Götzinger, et al.

Nanoskopische Quantensysteme in einem Festkörper finden in der Quantenoptik zunehmend an Bedeutung. Deren Integrierbarkeit in photonische Nanostrukturen machen sie zu aussichtsreichen Kandidaten zur Realisierung von zukünftigen Quantennetzwerken. Als Grundbaustein konnte kürzlich die effiziente Kopplung von einzelnen Molekülen an photonische Wellenleiterstrukturen gezeigt werden. Mit neuartigen Mikroresonatoren ist es möglich, die optische Kopplung zwischen einzelnen Quantensystemen zu untersuchen. Unterdessen kommen sogar einzelne Ionen in einem Kristall in der Nano-Quantenoptik zum Einsatz.

Nonlinear Plasmon Optics

Mario Hentschel, Tobias Utikal, Bernd Metzger, Harald Giessen, Markus Lippitz

We study nonlinear optics in plasmonic nanosystems, discuss the role of structural symmetries and the influence of linear optical properties. In particular, we investigate third harmonic generation from dimer nanoantennas and show that the nonlinear optical response, in contrast to common belief, is not governed by gap nonlinearities but fully described by the linear optical properties of the antenna. A simple nonlinear harmonic oscillator model is shown to reproduce all experimental features.

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.

Quantitative Modeling of the Third Harmonic Emission Spectrum of Plasmonic Nanoantennas

M. Hentschel, T. Utikal, H. Giessen, M. Lippitz

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.

Tailoring the photonic band splitting in metallodielectric photonic crystal superlattices

T. Utikal, T. Zentgraf, S. G. Tikhodeev, M. Lippitz, H. Giessen

We experimentally and theoretically investigate the influence of a structured supercell on the band splitting of one-dimensional metallodielectric photonic crystal superlattices. We show that the splitting of the photonic bands can be modified by periodic structuring of the elementary unit cell of the photonic crystal. For our investigation we constructed metallic photonic crystal superlattices by creating supercells from standard photonic crystal building blocks and arranged them at certain distances apart. The optical properties were obtained by conventional angle-resolved white-light transmission measurements.

Nonlinear photonics with metallic nanostructures on top of dielectrics and waveguides

T. Utikal, M. Hentschel, H. Giessen

We review recent experimental and theoretical studies of the ultrafast and nonlinear optical response of metallic nanostructures on top of dielectric substrates and slab waveguides where plasmon hybridization is a key ingredient. In a first three-pulse all-optical control experiment a hybrid plasmonic mode is turned on or off only a few tens of femtoseconds after its excitation. A second experiment concentrates on the origin of the nonlinear response in a metallo-dielectric photonic crystal structure. We show that the shape of the nonlinear optical spectra provides unambiguous information about the nonlinear optical contribution of the metallic as well as the dielectric part of the structure. Furthermore, we discuss the influence of slow-light on the nonlinear response. All experimental results agree perfectly with numerical scattering matrix calculations as well as simulations based on a classical harmonic oscillator model.

Towards the Origin of the Nonlinear Response in Hybrid Plasmonic Systems

T. Utikal, T. Zentgraf, T. Paul, C. Rockstuhl, F. Lederer, M. Lippitz, H. Giessen

Plasmonic systems are known for their distinct nonlinear optical properties when compared to purely dielectric materials. Although it is well accepted that the enhanced nonlinear processes in plasmonic-dielectric compounds are related to the excitation of localized plasmon resonances, their exact origin is concealed by the local field enhancement in the surrounding material and the nonlinearity in the metal. Here, we show that the origin of third-harmonic generation in hybrid plasmonic-dielectric compounds can be unambiguously identified from the shape of the nonlinear spectrum.

Excitonic Fano Resonance in Free-Standing Graphene

D. H. Chae, T. Utikal, S. Weisenburger, H. Giessen, K. von Klitzing, M. Lippitz, J. Smet

We investigate the role of electron-hole correlations in the absorption of free-standing monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common noninteracting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.

All-Optical Control of the Ultrafast Dynamics of a Hybrid Plasmonic System

T. Utikal, M. I. Stockman, A. P. Heberle, M. Lippitz, H. Giessen

Tailoring the ultrafast dynamics of the magnetic mode in magnetic photonic crystals

M. Geiselmann, T. Utikal, M. Lippitz, H. Giessen

Dynamics and dephasing of plasmon polaritons in metallic photonic crystal superlattices: Time- and frequency-resolved nonlinear autocorrelation measurements and simulations

T. Utikal, T. Zentgraf, J. Kuhl, H. Giessen

We present time- and frequency-resolved nonlinear autocorrelation measurements of polaritonic eigenstates in gold nanowire photonic crystal superlattices on a dielectric waveguide layer. The measurements show a complex behavior of the third-harmonic signal. The spectrum consists of several components with different intensities and time dynamics. Simulations based on a simple model, where the polaritonic eigenmodes are described by damped Lorentzian oscillators, show excellent agreement of time- and frequency-resolved data with the experiments. The simulations show that the superlattice structure leads to a strong modification of the polariton dynamics and prolonged dephasing times up to 60 fs compared to a simple lattice structure.