Observation of intrinsic size effects in the optical response of individual gold nanoparticles

The Photothermal Heterodyne Imaging method is used to study for the first time the absorption spectra of individual gold nanoparticles with diameters down to 5 nm. Intrinsic size effects wich result in a broadening of the Surface Plasmon resonance are unambiguously observed. Dispersions in the peak energies and homogeneous widths of the single particle resonances are revealed. The experimental results are analysed within the frame of Mie theory

Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals

Lead-halide perovskite nanocrystals (NCs) are attractive nano-building blocks for photovoltaics and optoelectronic devices as well as quantum light sources. Such developments require a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. In this review, we give an overview of recent magneto-optical spectroscopic studies revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of their inhomogeneities in morphology and crystal structure. We highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton–phonon coupling, which provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. We also decipher the spectroscopic characteristics of other charge complexes whose recombination contributes to photoluminescence. With the rich knowledge gained from these experiments, we provide some perspectives on perovskite NCs as quantum light sources

Indistinguishable near infra-red single photons from an individual organic molecule

By using the zero-phonon line emission of an individual organic molecule, we realized a source of indistinguishable single photons in the near infrared. A Hong-Ou-Mandel interference experiment is performed and a two-photon coalescence probability of higher than 50% at 2 K is obtained. The contribution of the temperature-dependent dephasing processes to the two-photon interference contrast is studied. We show that the molecule delivers nearly ideal indistinguishable single photons at the lowest temperatures when the dephasing is nearly lifetime limited. This source is used to generate post-selected polarization-entangled photon pairs, as a test-bench for applications in quantum information

Coherent Population Trapping of Single Spins in Diamond Under Optical Excitation

Coherent population trapping is demonstrated in single nitrogen-vacancy centers in diamond under optical excitation. For sufficient excitation power, the fluorescence intensity drops almost to the background level when the laser modulation frequency matches the 2.88 GHz splitting of the ground states. The results are well described theoretically by a four-level model, allowing the relative transition strengths to be determined for individual centers. The results show that all-optical control of single spins is possible in diamond.Comment: minor correction

Efficient biexciton emission in single CdSe nanocrystals

Quantum-confined nanoparticles have been increasingly investigated over the past decade due to the superior efficiency and tunability of their emission wavelength from the ultraviolet to the near infra red. Among those nanoparticles, colloidal CdSe nanocrystals (NC) are particularly attractive for many applications such as nanoscale electronics, laser technology, quantum cryptography, and biological fluorescent labeling. A detailed understanding of the NCs band-edge exciton fine structure is crucial for these applications. While intensive experimental and theoretical work has been performed to describe the size dependence of the exciton fine structure in nearly spherical NCs, the shape dependence has received much less attention despite recent advances in NC growth methods which lead to a greater control over shape distribution. Pioneering theoretical and experimental investigations have indicated that the shape dependence of NCs can be as important as the size dependence in terms of tuning their electronic and optical properties. The elucidation of these shape effects remains an experimental challenge which can be addressed by the optical study of individual NCs, where ensemble averaging over shape and size distributions is suppressed. Shape and size effects also govern the optical response of NCs in the multiexcitonic regime, where potential applications such as optical gain are envisaged. Despite the important role that biexcitons play in the optics of NCs, it has been practically impossible to observe the biexciton recombination line in the PL of CdSe NCs under continuous wave excitation, because of efficient nonradiative Auger recombination. This presentation will be focused on our recent magneto-optical and time-resolved spectroscopic investigations of single commercial qdot655 streptavidin conjugates NCs (comprising a core of CdSe capped by a ZnS layer) as a function of temperature. The remarkable photostability of these NCs at low temperature led us to unveil the spectral and temporal signatures of the emission from the lowest exciton-fine-structure states, trion emission and biexciton emission. Because of the NCs shape distribution, we find various band-edge exciton fine structures that are consistent with theoretical predictions for elongated NCs. Furthermore, contrarily to what was anticipated for "standard" CdSe-based core shell NCs, we show evidence for spectral and temporal signatures of highly efficient radiative biexcitonic recombinations in this type of NCs. Special attention will also be paid to the attractive trion (charged exciton) emission properties for potential applications in quantum information processing

Spectroscopie de nanocristaux de semiconducteur individuels

National audienceCours de deux heures filmé à l'école thématique "Nano-objets aux interfaces". Le cours, destiné en priorité à des doctorants ou des postdoctorants présente l'étude spectroscopique des nanocristaux de CdSe individuels, qui permet d'accéder à des informations précieuses sur la structure fine de l'exciton de bord de bande, les propriétés d'états biexcitoniques ou trioniques, en vue d'applications futures en information quantique

Magneto-optical spectroscopy of single CdSe nanocrystals

International audienc

Spectroscopie de nanocristaux de semiconducteur individuels

Cours de deux heures filmé à l'école thématique "Nano-objets aux interfaces". Le cours, destiné en priorité à des doctorants ou des postdoctorants présente l'étude spectroscopique des nanocristaux de CdSe individuels, qui permet d'accéder à des informations précieuses sur la structure fine de l'exciton de bord de bande, les propriétés d'états biexcitoniques ou trioniques, en vue d'applications futures en information quantique

Efficient biexciton emission from elongated CdSe/ZnS nanocrystals

Efficient biexciton emission from elongated CdSe/ZnS nanocrystals, Stuttgart Allemagne, 25 octobre 2011Seminar in the Physikalisches Institut, Universität Stuttgart. Invitation by Prof. J. Wrachtrup. The talk gives our latest results concerning microscopy and spectroscopy of single CdSe nanocrystals