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  4. Vahid Sandoghdar

Prof. Vahid Sandoghdar

  • Director
  • Head of Nano-Optics Division

The research of our group aims to advance experimental and theoretical mastery of light-matter interaction at the nanometer scale and to achieve the same degree of control and finesse that is known from the gas-phase quantum optics in the condensed phase. To do this, we combine concepts from quantum optics, laser spectroscopy, cryogenics, optical imaging, scanning probe technology and nanofluidics. In this endeavour, we have addressed a wide spectrum of scientific questions, ranging from quantum optics to biophysics. For more information, please consult our research website and our list of publications.

2014

High-cooperativity nanofiber laser

Sanli Faez, Pierre Türschmann, Vahid Sandoghdar

arXiv 1403.1885 (2014) | Preprint | PDF

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

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

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

Biomaterials Science 2 883-892 (2014) | Journal

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

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

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

Optica 1 203-208 (2014) | Journal

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

Nature Communications 5 3627 (2014) | Journal

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.

Scanning-aperture trapping and manipulation of single charged nanoparticles

Ji Tae Kim, Susann Spindler, Vahid Sandoghdar

Nature Communications 5 3380 (2014) | Journal

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

Cryogenic Colocalization Microscopy for Nanometer-Distance Measurements

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

SI 15 763-770 (2014) | Journal

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

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

Marek Piliarik, Vahid Sandoghdar

Nature Communications 5 4495 (2014) | Journal

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

Single-molecule optical spectroscopy

Michel Orrit, Taekjip Ha, Vahid Sandoghdar

Chemical Society Reviews 43 973-976 (2014) | Journal

Coherent Interaction of Light and Single Molecules in a Dielectric Nanoguide

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

Physical Review Letters 113 213601 (2014) | Journal

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.

Label-free characterization of biomembranes: from structure to dynamics

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

Chemical Society Reviews 43 887-900 (2014) | Journal

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

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

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

The Journal of Physical Chemistry B 118 1545-1554 (2014) | Journal

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

Born on April 29, 1966 in Tehran, Iran. Bachelor of Science in Physics from the University of California in Davis (1987), Ph.D. in Physics (supervisors: E. A. Hinds and S. Haroche) from Yale University (1993), Postdoctoral Fellow at École Normale Supérieure (group of S. Haroche) in Paris. Head of the Nano-Optics group und habilitation in Physics at University of Konstanz (Chair of J. Mlynek). Professorship at Eidgenössischen Technischen Hochschule (ETH) Zurich (2001-2011). Recipient of an ERC Advanced Grant (2010). Alexander von Humboldt Professorship at Friedrich-Alexander-Universität Erlangen-Nürnberg and Director and Scientific Member at the Max Planck Institute for the Science of Light in Erlangen since 2011. Fellow of the Optical Society (OSA) and recepient of the 2023 Quantum Electronics and Optics Award for Fundamental Aspects from the European Physical Society. Founder of the Max-Planck-Zentrum für Physik und Medizin, a joint research center that aims to address questions in fundamental medical research with physical and mathematical methods.

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