Ref HAL: tel-02476502_v1
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Ce travail porte sur l’étude optique de la dynamique de spin de deux systèmes: un gaz d’électrons dans des couches minces de CdTe d’une part et des excitons indirects dans un double puits quantique asymétrique en GaAs d’autre part. Des mesures de photoluminescence résolue en temps et en polarisation, et des mesures de spectroscopie pompe-sonde ont permis la détermination des temps de vie et des temps de relaxation de spin des excitons indirects. Le comportement général de la structure a été décrit, les contraintes techniques ont été mise en évidence et les meilleures conditions expérimentales ont été identifiées. En photoluminescence, nous avons mesuré des temps de vie de l’ordre de la quinzaine de ns et des temps de relaxation de spin de 5 ns dans le meilleur cas. L’utilisation d’un setup de spectroscopie pompe-sonde permettant d’étudier des délais très longs a démontré que des temps plus longs encore peuvent être atteints en séparant d’avantage deux impulsions lasers successives.Pour les électrons dans CdTe nous avons utilisé une autre méthode optique: la spectroscopie de bruit de spin qui s’est récemment imposée comme un outil de choix pour étudier la dynamique de spin dans les semi-conducteurs. Son principe consiste à sonder la dynamique d’un système de spins à travers ses fluctuations spontanées. Pour ce faire, ces fluctuations sont encodées dans le plan de polarisation d’un laser hors résonnant par l’intermédiaire de la rotation Faraday.Alors que les réalisations concrètes de cette technique se limitaient jusqu’à présent aux corrélations temporelles, nous proposons ici la première implémentation permettant d’accéder également aux corrélations spatiales du systèmes de spin. Cet accès à la dynamique spatiale est autorisé par une sélectivité en vecteur d’onde de la lumière diffusée venant de l’échantillon et nous offre l’opportunité de mesurer simultanément le temps de relaxation de spin et le coefficient de diffusion de spin. Ayant ainsi une vision complète de la dynamique de spin dans CdTe, nous avons pu confronter la physique du spin bien connue dans GaAs à nos observations dans CdTe. Contre toutes attentes, il semblerait que nos connaissances de GaAs ne soient pas directement transposables à CdTe.

Ref HAL: hal-02466887_v1
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Dipolar, or indirect excitons (IXs) offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. Once confined, the exciton fluid accumulates in uniform zones. By modeling the emission spectra of these zones, we get insight into the thermodynamic properties of the thermalized exciton fluid. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02466858_v1
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Résumé:

Dipolar, or indirect excitons (IXs) offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures UP TO ROOM TEMPERATURES, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02393815_v1
DOI: 10.1038/s41598-019-54416-3
WoS: 000500557100001
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6 Citations
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On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Ref HAL: hal-02272702_v1
Ref Arxiv: 1902.02974
DOI: 10.1021/acs.nanolett.9b00914
WoS: 000481563800013
Ref. & Cit.: NASA ADS
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1 Citation
Résumé:

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02192421_v1
DOI: 10.1103/PhysRevLett.123.017401
WoS: WOS:000473540500009
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3 Citations
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We report on the potential of a new spin noise spectroscopy approach by demonstrating all-optical probing of spatiotemporal spin fluctuations. This is achieved by homodyne mixing of a spatially phase-modulated local oscillator with spin-flip scattered light, from which the frequency and wave vector dependence of the spin noise power is unveiled. As a first application of the method we measure the spatiotemporal spin noise in weakly n-doped CdTe layers, from which the electron spin diffusion constant and spin relaxation rates are determined. The absence of spatial spin correlations is also shown for this particular system.

Ref HAL: hal-02142651_v1
DOI: 10.1103/PhysRevB.99.205305
WoS: WOS:000467726600010
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2 Citations
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Faraday rotation, being a dispersive effect, is commonly considered as the method of choice for nondestructive detection of spin states. Nevertheless Faraday rotation is inevitably accompanied by spin flips induced by Raman scattering, which compromises nondestructive detection. Here, we derive an explicit general relation relating the Faraday rotation and the spin-flip Raman scattering cross sections, from which precise criteria for nondestructive detection are established. It is shown that, even in ideal conditions, nondestructive measurement of a single spin can be achieved only in anisotropic media. Monolayers of transitions metal dichalcogenides are shown to be promising candidates for this purpose.