Prof. Vahid Sandoghdar

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

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

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

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).

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