Giant Quantum Electrodynamic Effects on Single SiV Color Centers in Nanosized Diamonds.

Understanding and mastering quantum electrodynamics phenomena is essential to the development of quantum nanophotonics applications. While tailoring of the local vacuum field has been widely used to tune the luminescence rate and directionality of a quantum emitter, its impact on their transition energies is barely investigated and exploited. Fluorescent defects in nanosized diamonds constitute an attractive nanophotonic platform to investigate the Lamb shift of an emitter embedded in a dielectric nanostructure with high refractive index.

High-resolution optical imaging of single magnetic flux quanta with a solid immersion lens.

Magneto-optical imaging of quantized magnetic flux tubes in superconductors - Abrikosov vortices - is based on Faraday rotation of light polarization within a magneto-optical indicator placed on top of the superconductor. Due to severe aberrations induced by the thick indicator substrate, the spatial resolution of vortices is usually well beyond the optical diffraction limit. Using a high refractive index solid immersion lens placed onto the indicator garnet substrate, we demonstrate wide field optical imaging of single flux quanta in a Niobium film with a resolution better than 600 nm and sub-second acquisition periods, paving the way to high-precision and fast vortex manipulation.

Revealing the Band-Edge Exciton Fine Structure of Single InP Nanocrystals.

We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0 with zero angular momentum projection along the NC main elongation axis.

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites.

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies.

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.

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots.

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features.

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.

The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals.

Cesium lead halide perovskites exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. Yet, the physics of the band-edge exciton, whose recombination is at the origin of the photoluminescence, is not elucidated. Here, we unveil the exciton fine structure of individual cesium lead iodide perovskite nanocrystals and demonstrate that it is governed by the electron-hole exchange interaction and nanocrystal shape anisotropy.

On-Demand Optical Generation of Single Flux Quanta.

Superconductors can host quantized magnetic flux tubes surrounded by supercurrents, called Abrikosov vortices. Vortex penetration into a superconducting film is usually limited to its edges and triggered by external magnetic fields or local electrical currents. With a view to novel research directions in quantum computation, the possibility to generate and control single flux quanta in situ is thus challenging.

Memories in the photoluminescence intermittency of single cesium lead bromide nanocrystals.

Single cesium lead bromide (CsPbBr) nanocrystals show strong photoluminescence intermittency, with on- and off- dwelling times following power-law distributions. We investigate the correlations for successive on-times and successive off-times, and find a memory effect in the photoluminescence intermittency of such inorganic perovskite nanocrystals. This memory effect is not sensitive to the nature of the surface capping ligand and the embedding polymer.

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.

The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state.

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains unknown and debated. While symmetry analysis predicts a singlet non-emissive ground exciton topped with a bright exciton triplet, it has been predicted that the Rashba effect may reverse the bright and dark level ordering.

Chemical Cutting of Perovskite Nanowires into Single-Photon Emissive Low-Aspect-Ratio CsPbX (X=Cl, Br, I) Nanorods.

Post-synthetic shape-transformation processes provide access to colloidal nanocrystal morphologies that are unattainable by direct synthetic routes. Herein, we report our finding about the ligand-induced fragmentation of CsPbBr perovskite nanowires (NWs) into low aspect-ratio CsPbX (X=Cl, Br and I) nanorods (NRs) during halide ion exchange reaction with PbX -ligand solution. The shape transformation of NWs-to-NRs resulted in an increase of photoluminescence efficiency owing to a decrease of nonradiative decay rates.

Unraveling exciton-phonon coupling in individual FAPbI nanocrystals emitting near-infrared single photons.

Formamidinium lead iodide (FAPbI) exhibits the narrowest bandgap energy among lead halide perovskites, thus playing a pivotal role for the development of photovoltaics and near-infrared classical or quantum light sources. Here, we unveil the fundamental properties of FAPbI by spectroscopic investigations of nanocrystals of this material at the single-particle level. We show that these nanocrystals deliver near-infrared single photons suitable for quantum communication.

A solid state source of photon triplets based on quantum dot molecules.

Producing advanced quantum states of light is a priority in quantum information technologies. In this context, experimental realizations of multipartite photon states would enable improved tests of the foundations of quantum mechanics as well as implementations of complex quantum optical networks and protocols. It is favourable to directly generate these states using solid state systems, for simpler handling and the promise of reversible transfer of quantum information between stationary and flying qubits.

Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy.

Revealing the crystal structure of lead halide perovskite nanocrystals is essential for the optimization of stability of these emerging materials in applications such as solar cells, photodetectors, and light-emitting devices. We use magneto-photoluminescence spectroscopy of individual perovskite CsPbBr nanocrystals as a unique tool to determine their crystal structure, which imprints distinct signatures in the excitonic sublevels of charge complexes at low temperatures. At zero magnetic field, the identification of two classes of photoluminescence spectra, displaying either two or three sublevels in their exciton fine structure, shows evidence for the existence of two crystalline structures, namely tetragonal D and orthorhombic D phases.

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.

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.

Tailoring the exciton fine structure of cadmium selenide nanocrystals with shape anisotropy and magnetic field.

We use nominally spheroidal CdSe nanocrystals with a zinc blende crystal structure to study how shape perturbations lift the energy degeneracies of the band-edge exciton. Nanocrystals with a low degree of symmetry exhibit splitting of both upper and lower bright state degeneracies due to valence band mixing combined with the isotropic exchange interaction, allowing active control of the level splitting with a magnetic field. Asymmetry-induced splitting of the bright states is used to reveal the entire 8-state band-edge fine structure, enabling complete comparison with band-edge exciton models.

State selective pumping reveals spin-relaxation pathways in CdSe quantum dots.

The band-edge exciton in elongated CdSe nanocrystals is composed of an upper and lower manifold associated with heavy and light holes in which the energy separation is sensitive to the nanocrystal shape. Using resonant photoluminescence excitation, we probe the upper heavy hole exciton manifold and find rapid relaxation to the lower light hole manifold on a 5 ps time scale. State selective excitation allows the preparation of single quantum states in this system.

The optical phonon spectrum of CdSe colloidal quantum dots.

The direct coupling of excited electronic states to optical phonons in single CdSe colloidal quantum dots is explored using both photoluminescence emission and excitation spectroscopies. We find a broad optical phonon spectrum associated with a single fine structure state. Multiple peaks in the optical phonon sideband are ascribed to different optical phonon types emanating from both the core and shell layers.

The ultimate limit to the emission linewidth of single nanocrystals.

Measurements of the emission linewidth of single nanocrystals are usually limited by spectral diffusion. At cryogenic temperatures, the origin of this instability was revealed to be photo-induced, suggesting that the spectral peak position may be stable in the limit of vanishing optical excitation. Here we test this stability using resonant photoluminescence excitation and find there is persistent spectral broadening, which ultimately limits the emission linewidth in these materials.

Spectroscopy of single nanocrystals.

As colloidal semiconductor nanocrystals are developed for a wider range of diverse applications, it becomes more important to gain a deeper understanding of their properties in order to direct synthetic efforts. While most synthetic developments are guided by changes in ensemble properties, certain applications such as those in nano-electronics and nano-photonics rely on properties of nanocrystals at the individual level. For such applications and even for a more detailed understanding of the ensemble behavior, single nanocrystal spectroscopy becomes a vital tool.

Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots.

Spectroscopically resolved emission from single nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures, the emission spectra collapse to narrow lines, revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were first used, the technology of nanocrystal synthesis has matured significantly, and new materials with outstanding photostability have been reported.