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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. Using spectral and time-resolved optical spectroscopy of single SiV defects, we unveil blue shifts (up to 80 meV) of their emission lines, which are interpreted from model calculations as giant Lamb shifts. Moreover, evidence for a positive correlation between their fluorescence decay rates and emission line widths is observed, as a signature of modifications not only of the photonic local density of states but also of the phononic one, as the nanodiamond size is decreased. Correlative light-electron microscopy of single SiVs and their host nanodiamonds further supports these findings. These results make nanodiamond-SiVs promising as optically driven spin qubits and quantum light sources tunable through nanoscale tailoring of vacuum-field fluctuations.
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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 600â nm and sub-second acquisition periods, paving the way to high-precision and fast vortex manipulation. Vectorial field simulations are also performed to reproduce and optimize the experimental features of vortex images.
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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 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 µeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity.
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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. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.
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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. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.
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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.
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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. The lowest-energy exciton state is a long-lived dark singlet state, which promotes the creation of biexcitons at low temperatures and thus correlated photon pairs. These bright quantum emitters in the near-infrared have a photon statistics that can readily be tuned from bunching to antibunching, using magnetic or thermal coupling between dark and bright exciton sublevels.
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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. We introduce a far-field optical method to sculpt the magnetic flux or generate permanent single vortices at any desired position in a superconductor. It is based on a fast quench following the absorption of a tightly focused laser pulse that locally heats the superconductor above its critical temperature. We achieve ex-nihilo creation of a single vortex pinned at the center of the hotspot, while its counterpart opposite flux is trapped tens of micrometers away at its boundaries. Our method paves the way to optical operation of Josephson transport with single flux quanta.
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Single cesium lead bromide (CsPbBr3) 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. These observations suggest that photoluminescence intermittency and its memory are mainly controlled by intrinsic traps in the nanocrystals. Our findings will help optimizing light-emitting devices based on these perovskite nanocrystals.
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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. Here, we provide the direct spectroscopic signature of the dark exciton emission in the low-temperature photoluminescence of single formamidinium lead bromide perovskite nanocrystals under magnetic fields. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation.
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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 CsPbBr3 perovskite nanowires (NWs) into low aspect-ratio CsPbX3 (X=Cl, Br and I) nanorods (NRs) during halide ion exchange reaction with PbX2 -ligand solution. The shape transformation of NWs-to-NRs resulted in an increase of photoluminescence efficiency owing to a decrease of nonradiative decay rates. Importantly, we found that the perovskite NRs exhibit single photon emission as revealed by photon antibunching measurements, while it is not detected in parent NWs. This work not only reports on the quantum light emission of low aspect ratio perovskite NRs, but also expands our current understanding of shape-dependent optical properties of perovskite nanocrystals.
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Formamidinium lead iodide (FAPbI3) 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 FAPbI3 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. Moreover, the low temperature photoluminescence spectra of FAPbI3 nanocrystals reveal the optical phonon modes responsible for the emission line broadening with temperature and a vanishing exciton-acoustic phonon interaction in these soft materials. The photoluminescence decays are governed by thermal mixing between fine structure states, with a two-optical phonon Raman scattering process. These results point to a strong Frölich interaction and to a phonon glass character that weakens the interactions of charge carriers with acoustic phonons and thus impacts their relaxation and mobility in these perovskites.
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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. Here we use the ground states of two optically active coupled quantum dots to directly produce photon triplets. The formation of a triexciton in these ground states leads to a triple cascade recombination and sequential emission of three photons with strong correlations. We record 65.62 photon triplets per minute under continuous-wave pumping, surpassing rates of earlier reported sources. Our structure and data pave the way towards implementing multipartite photon entanglement and multi-qubit readout schemes in solid state devices.
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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 CsPbBr3 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 D4h and orthorhombic D2h phases. Magnetic field shifts, splitting, and coupling of the sublevels provide a determination of the diamagnetic coefficient and valuable information on the exciton g-factor and its anisotropic character. Moreover, this spectroscopic study reveals the optical properties of charged excitons and allows the extraction of the electron and hole g-factors for perovskite systems.
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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.
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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. We used this to map the hole spin relaxation pathways between the fine structure sublevels, which have energy splittings incommensurate with either optical or acoustic phonon energies. This reveals a hitherto unexpected hole spin-relaxation channel in these materials.
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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. A mixed emission process that involves the simultaneous generation of two different types of optical phonon is also observed. In general, rather than a single mode, each designated phonon type is associated with a dispersed family of modes. Narrow optical phonon sidebands, consistent with the dominant LO mode, are observed in some nanocrystals. A linewidth analysis indicates that optical phonon lifetimes are in the 10 picosecond range. We demonstrate the ability to selectively excite a specific band-edge state by directly exciting its LO phonon sideband.
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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. This review looks at how single nanocrystal spectroscopy has been applied to materials based on modern synthetic techniques and how these studies are elucidating properties that remain hidden at the ensemble level. First, recent theoretical models that are important for understanding many observed phenomena are explored. Then the review highlights new insights into many of the photophysical properties that are of interest in semiconductor nanocrystal materials, such as the ubiquitous spectral instability, magneto-optical identification of the band-edge exciton fine structure, emission from multi-excitons, and the spectroscopic properties of charged nanocrystals that challenge long standing theories on photoluminescence blinking behavior. To date most of the research has been conducted on materials based on cadmium selenide primarily due to its many years of development as a prototypical nanocrystal system. The review ends with a discussion of new materials that would also benefit from a detailed photophysical understanding afforded by single nanocrystal spectroscopy.
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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. The spectral broadening is shown to be consistent with spontaneous fluctuations of the local electrostatic field within the disordered environment surrounding the nanocrystal.