Tailoring the superradiant and subradiant nature of two coherently coupled quantum emitters.

The control and manipulation of quantum-entangled states is crucial for the development of quantum technologies. A promising route is to couple solid-state quantum emitters through their optical dipole-dipole interactions. Entanglement in itself is challenging, as it requires both nanometric distances between emitters and nearly degenerate electronic transitions. Here we implement hyperspectral imaging to identify pairs of coupled dibenzanthanthrene molecules, and find distinctive spectral signatures of maximally entangled superradiant and subradiant electronic states by tuning the molecular optical resonances with Stark effect. We demonstrate far-field selective excitation of the long-lived subradiant delocalized state with a laser field tailored in amplitude and phase. Optical nanoscopy of the coupled molecules unveils spatial signatures that result from quantum interferences in their excitation pathways and reveal the location of each emitter. Controlled electronic-states superposition will help deciphering more complex physical or biological mechanisms governed by the coherent coupling and developing quantum information schemes.

Inverse Faraday Effect for Superconducting Condensates.

The Cooper pairs in superconducting condensates are shown to acquire a temperature-dependent dc magnetic moment under the effect of the circularly polarized electromagnetic radiation. The mechanisms of this inverse Faraday effect are investigated within the simplest version of the phenomenological dynamic theory for superfluids, namely, the time-dependent Ginzburg-Landau (GL) model. The light-induced magnetic moment is shown to be strongly affected by the nondissipative oscillatory contribution to the superconducting order parameter dynamics, which appears due to the nonzero imaginary part of the GL relaxation time. The relevance of the latter quantity to the Hall effect in the superconducting state allows us to establish the connection between the direct and inverse Faraday phenomena.

3D optical nanoscopy with excited state saturation at liquid helium temperatures.

We present a 3D fluorescence nanoscopy method operating at cryogenic temperatures, based on optical saturation of the excited state of individual molecules. Using a focused laser beam structured with a zero-intensity central region surrounded by intensity gradients in the three space directions, we achieve a sub-30 nm 3D optical resolution. Moreover, the analysis of the fluorescence scanning images of single molecules reveals the 3D orientation of their transition dipole with an accuracy of a few degrees. This method provides a valuable tool for locating neighboring molecules with overlapping optical transitions in order to study their interactions.

Optical manipulation of single flux quanta.

Magnetic field can penetrate into type II superconductors in the form of Abrikosov vortices, which are magnetic flux tubes surrounded by circulating supercurrents often trapped at defects referred to as pinning sites. Although the average properties of the vortex matter in superconductors can be tuned with magnetic fields, temperature or electric currents, handling of individual Abrikosov vortices remains challenging and has been demonstrated only with sophisticated scanning local probe microscopies. Here we introduce a far-field optical method based on local heating of the superconductor with a focused laser beam to realize a fast and precise manipulation of individual vortices, in the same way as with optical tweezers. This simple approach provides the perfect basis for sculpting the magnetic flux profile in superconducting devices like a vortex lens or a vortex cleaner, without resorting to static pinning or ratchet effects.

Polarization effects in lattice-STED microscopy.

Massive parallelization of STED-like nanoscopies is now achievable using well-designed optical lattices for state depletion. Yet, only the lattice intensity distribution was considered for the description of the super-resolved point spread function. This holds for fast-rotating fluorescent emitters. Here, we study the effects of electric field topography in lattice-STED microscopy. The dependence of the super-resolved point spread function on the number of dipoles and their orientation is investigated. Single fluorescent nano-diamonds are imaged using different optical lattice configurations and the measured resolutions are compared to theoretical simulations.

Direct Evidence of Flexomagnetoelectric Effect Revealed by Single-Molecule Spectroscopy.

We report direct evidence of the electric field induced by a magnetization inhomogeneity in an iron garnet film. This inhomogeneity was created by the nonuniform magnetic fields generated at domain boundaries of a type-I superconductor in the intermediate state. At liquid helium temperatures, Stark shifts of sharp single-molecule zero-phonon lines were used to probe the local electric fields generated by this flexomagnetoelectric effect. The measured electric fields are in accordance with theoretical estimations.

Efficient biexciton emission in elongated CdSe/ZnS nanocrystals.

We use a combination of low-temperature magneto-optical and lifetime spectroscopies to study the band-edge exciton fine structure of highly photostable single CdSe/ZnS nanocrystals (NCs). Neutral NCs displaying multiline emission spectra and multiexponential photoluminescence (PL) decays are studied as a function of temperature and external magnetic fields. Three different fine structure regimes are identified as a function of the NC aspect ratio. In particular, we identify an optically inactive ground exciton state, whose oscillator strength is tuned up under magnetic field coupling to bright exciton states, and attribute it to the zero angular momentum ground exciton state of elongated NCs. We also show evidence for highly efficient biexciton emission in these NCs, with radiative yields approaching unity in some cases.

Band-edge exciton fine structure of single CdSe/ZnS nanocrystals in external magnetic fields.

We report a spectroscopic study of the two lowest-energy exciton levels of individual CdSe/ZnS nanocrystals under applied magnetic fields. Field-induced coupling between the bright and the dark excitonic states is directly observed in the low-temperature photoluminescence spectrum and decay and allows the determination of the angle between the nanocrystal c axis and the field. Orientation-dependent Zeeman splittings of the dark and bright exciton sublevels are measured and provide the corresponding exciton Landé factors, as well as spin-flip relaxation rates between Zeeman sublevels.

Direct observation of the two lowest exciton zero-phonon lines in single CdSe/ZnS nanocrystals.

We report a spectroscopic study of highly photostable individual CdSe/ZnS colloidal nanocrystals. At low temperature, photoluminescence spectra display two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest levels of the band-edge exciton fine structure. For the first time, resonant photoluminescence excitation spectra of these lines is performed, and spectral diffusion broadening of 10 microeV is measured over integration times of 100 ms, corresponding to an optical coherence lifetime longer than 100 ps.

Efficient generation of near infra-red single photons from the zero-phonon line of a single molecule.

Using the zero-phonon line (ZPL) emission of a single molecule, we realized a triggered source of near-infra-red (lambda = 785 nm) single photons at a high repetition rate. A Weierstrass solid immersion lens is used to image single molecules with an optical resolution of 300 nm (approximately 0.4lambda) and a high collection efficiency. Because dephasing of the transition dipole due to phonons vanishes at liquid helium temperatures, our source is attractive for the efficient generation of single indistinguishable photons.

Velocity profiles of water flowing past solid glass surfaces using fluorescent nanoparticles and molecules as velocity probes.

Measurements of the velocity profile of water flowing on a glass surface using fluorescent nanoparticles and single fluorescent molecules as velocity probes show that the no slip boundary condition holds down to at least 10 nm from the surface. For water flowing on a hydrophobic solid surface, silanized glass, the no slip boundary condition fails, and a slip length of 45 nm is measured. These velocity measurements are complemented with atomic force microscopy measurements of dissipation on a small sphere oscillating near the surface with results in agreement with the velocity profiles.

Drag enhancement with polymers.

Drag on a cylinder can be enhanced in the presence of polymers. This enhancement is directly related to the interaction of the polymers with the flow around the obstacle. The presence of the polymers modifies the flow in the vicinity of the cylinder giving rise to a band of higher shear, large molecular elongations and large velocity fluctuations. The measured drag on the cylinder is directly related to the modification of the flow field by the presence of the polymers.

Laser-induced resonance shifts of single molecules self-coupled by a metallic surface.

The spectral properties of single molecules placed near a metallic surface are investigated at low temperatures. Because of the high quality factor of the optical resonance, a laser-induced shift of the molecular lines is evidenced for the first time. The shift dependence on the laser excitation intensity and on the dephasing rate of the transition dipole is studied. A simple theoretical model of a laser-driven molecule self-coupled by a mirror is developed to qualitatively interpret the observations.

Absorption and scattering microscopy of single metal nanoparticles.

Several recently developed detection techniques opened studies of individual metal nanoparticles (1-100 nm in diameter) in the optical far field. Eliminating averaging over the broad size and shape distributions produced by even the best of current synthesis methods, these studies hold great promise for gaining a deeper insight into many of the properties of metal nanoparticles, notably electronic and vibrational relaxation. All methods are based on detection of a scattered wave emitted either by the particle itself, or by its close environment. Direct absorption and interference techniques rely on the particle's scattering and have similar limits in signal-to-noise ratio. The photothermal method uses a photo-induced change in the refractive index of the environment as an additional step to scatter a wave with a different wavelength. This leads to a considerable improvement in signal-to-background ratio, and thus to a much higher sensitivity. We briefly discuss and compare these various techniques, review the new results they generated so far, and conclude on their great potential for nanoscience and for single-molecule labelling in biological assays and live cells.

Single metallic nanoparticle imaging for protein detection in cells.

We performed a visualization of membrane proteins labeled with 10-nm gold nanoparticles in cells, using an all-optical method based on photothermal interference contrast. The high sensitivity of the method and the stability of the signals allows 3D imaging of individual nanoparticles without the drawbacks of photobleaching and blinking inherent to fluorescent markers. A simple analytical model is derived to account for the measurements of the signal amplitude and the spatial resolution. The photothermal interference contrast method provides an efficient, reproducible, and promising way to visualize low amounts of proteins in cells by optical means.

Single photons on demand from a single molecule at room temperature.

The generation of non-classical states of light is of fundamental scientific and technological interest. For example, 'squeezed' states enable measurements to be performed at lower noise levels than possible using classical light. Deterministic (or triggered) single-photon sources exhibit non-classical behaviour in that they emit, with a high degree of certainty, just one photon at a user-specified time. (In contrast, a classical source such as an attenuated pulsed laser emits photons according to Poisson statistics.) A deterministic source of single photons could find applications in quantum information processing, quantum cryptography and certain quantum computation problems. Here we realize a controllable source of single photons using optical pumping of a single molecule in a solid. Triggered single photons are produced at a high rate, whereas the probability of simultaneous emission of two photons is nearly zero--a useful property for secure quantum cryptography. Our approach is characterized by simplicity, room temperature operation and improved performance compared to other triggered sources of single photons.

Ten Years of Single-Molecule Spectroscopy.

Single-molecule spectroscopy (SMS) combines some of the advantages of local probe microscopies with those of optics. Since this field came into being 10 years ago, it has expanded at a breathtaking pace. From the first cryogenic experiments up to the recent studies of basic processes in molecular biology, single-molecule methods have found their way into an ever broadening range of applications. Their common feature is the complete elimination of ensemble averaging. By exposing individual variations as well as dynamical fluctuations, SMS provides new insights into any system with spatial or temporal inhomogeneity. The present article illustrates single molecule spectroscopic experiments at cryogenic temperatures, mainly from the authors' group. The results reviewed here range from molecular photophysics, to the dynamics of the solid matrix around the molecule, and to the interactions between a single molecule and electromagnetic fields, i.e., quantum optics. SMS is now ripe for a variety of applications in physical chemistry, such as, for example, surfaces, growth structures, catalysis, or porous media.