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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.

2006

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

Alois Renn, Johannes Seelig, Vahid Sandoghdar

Molecular Physics 104 409-414 (2006) | Journal

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

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

Nano Letters 6 1151-1154 (2006) | Journal

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.

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

Sergei Kuehn, Ulf Hakanson, Lavinia Rogobete, Vahid Sandoghdar

Physical Review Letters 97 017402 (2006) | Journal

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

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

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

Optics Express 14 405-414 (2006) | Journal

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

Modification of single molecule fluorescence by a scanning probe

S. Kuehn, Vahid Sandoghdar

Applied Physics B: Lasers and Optics 84 211-217 (2006) | Journal

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

Optical detection of very small nonfluorescent nanoparticles

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

Chimia 60 A761-A764 (2006) | Journal

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

Optical detection of very small nonfluorescent nanoparticles

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

Chimia 60 A761-A764 (2006) | Journal

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

Measurement of the complex dielectric constant of a single gold nanoparticle

Patrick Stoller, Volker Jacobsen, Vahid Sandoghdar

Optics Letters 31 2474-2476 (2006) | Journal

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

Spontaneous emission rates of dipoles in photonic crystal membranes

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

Journal of the Optical Society of America B-Optical Physics 23 1196-1206 (2006) | Journal

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

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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