We report on the dynamics of electron spins in n-doped CdTe layers that differs significantly from the expected response derived from the studies dedicated to electron spin relaxation in n-GaAs. At zero magnetic field, the electron spin noise spectra exhibit a two-peak structure-a zero-frequency line and a satellite-that we attribute to the electron spin precession in a frozen random nuclear spin distribution. This implies a surprisingly long electron spin correlation time whatever the doping level, even above the Mott transition. Using spatiotemporal spin noise spectroscopy, we demonstrate that the observation of a satellite in the spin noise spectra and a fast spin diffusion are mutually exclusive. This is consistent with a shortening of the electron spin correlation time due to hopping between donors. We interpret our data via a model assuming that the low temperature spin relaxation is due to hopping between donors in presence of hyperfine and anisotropic exchange interactions. Most of our results can be interpreted in this framework. First, a transition from inhomogeneous to homogeneous broadening of the spin noise peaks and the disappearance of the satellite are observed when the hopping rate becomes larger than the Larmor period induced by the local nuclear fields. In the regime of homogeneous broadening the ratio between the spin diffusion constant and the spin relaxation rate has a value in good agreement with the Dresselhaus constant. In the regime of inhomogeneous broadening, most of the samples exhibit a broadening consistent with the distribution of local nuclear fields. We obtain a new estimate of the hyperfine constants in CdTe and a value of 0.10 Tesla for the maximum nuclear field. Finally, our study also reveals a puzzle as our samples behave as if the active donor concentration was reduced by several orders of magnitudes with respect to the nominal values.

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.

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.

Density and spin dynamics of indirect excitons in asymmetric GaAs/AlGaAs coupled quantum wells is studied by time-resolved photoluminescence. Under electric bias applied in the growth direction the lifetime of indirect excitons formed by an electron in a wide and hole in a narrow quantum well reaches 12 ns, while the circular polarisation lifetime is about 5 ns. The structure is suited for further studies of dark indirect excitons by time-resolved pump-probe spectroscopy

spin fluctuations in atomic vapors and in semiconductors. As such it is quite attractive for probing non-perturbatively electrons or nuclear spins in atomic vapours or in semiconductors, particularly when the laser is detuned from optical resonances, an aspect which has triggered considerable interest to spin noise spectroscopy in recent years. The signal in spin noise spectroscopy can be considered as arising from intensity fluctuations due to the interference between the light scattered by Raman spin-flip excitations and the probe laser. This suggests that homodyne or heterodyne detection of spin noise by mixing of the Raman signal with a local oscillator is feasible, which we have demonstrated recently [1]. This opportunity is essential to spin noise spectroscopy because it allows us to increase its sensitivity and accessible frequency range.Homodyne and/or heterodyne detection of spin noise for scattered light directions different from that of the probe should give access to spatio-temporal spin correlations [2], and therefore could considerably extend the application range of spin noise spectroscopy. Here we demonstrate non-collinear homodyne detection of spin noise in n-doped CdTe epilayers. This allows us to exploit different scattering geometries based on spin-flip Raman selection rules. We are able to measure simultaneously different fluctuating spin components by homodyne mixing in different space directions, and also for different local oscillator polarizations. [1] S. Cronenberger and D. Scalbert, Review of Scientific Instruments 87, 093111 (2016).[2] G. G. Kozlov, I. I. Ryzhov, and V. S. Zapasskii, Phys. Rev. A 95, 043810 (2017).

In the recent years the spectroscopy of spin noise has won its spurs for spin dynamics related studies in semiconductors, largely because it exhibits several quite attractive features. Noticeably it enables almost perturbation-free detection of spontaneous electron spin fluctuations. In addition as the amplitude of spin noise grows when the size of the probed region is reduced, it is well adapted for spatially resolved studies. Also the spin noise spectrum is quite sensitive to internal effective fields, which allows to probe locally the existence of nuclear fields. Finally, the combination of spin noise spectroscopy and optical heterodyning has been demonstrated, which permits enhanced sensitivity and broadband detection, while keeping a high spectral resolution [1].In this poster we will present results obtained by heterodyne detection of spin noise in an n-doped CdTe epilayer with donor density n∼3×〖10〗^17cm-3. Thanks to the enhanced sensitivity gained by heterodyne detection we could detect the spin noise of electrons bound to neutral donors for probe powers as low as 5 µW. In these conditions we observe an extremely slow hopping rate W0 of electron spin between neighbouring donors, and extremely slow electron spin relaxation rate s (see Figure 1). The observed noise spectrum at zero field is characteristic of electron spin precession in the frozen nuclear field acting on the electrons bound to the donors and exhibits two components. A central lorentzian line corresponding to the electron spin component along the nuclear field, and two satellites gaussian lines corresponding to the spin precession in the nuclear field. We analyze our results in the framework of a theoretical model, which takes into account both the electron spin precession, and the hopping between donors, but slightly modified to take into account an eventual non-zero nuclear spin polarization [2]. In agreement with the theory we can see that the satellites merge with the central line as the hopping rate increases (at the highest probe power). Surprisingly W0 and s become quite small at the lowest probe power. We find that the electron spin relaxation time becomes longer than 1 µs in this regime.

We show that graphene-dielectric multilayers give rise to an unusual tunability of the Casimir-Lifshitz forces and allow to easily realize completely different regimes within the same structure. Concerning thermal effects, graphene-dielectric multilayers take advantage of the anomalous features predicted for isolated suspended graphene sheets, even though they are considerably affected by the presence of the dielectric substrate. They can also achieve the anomalous nonmonotonic thermal metallic behavior by increasing the graphene sheets density and their Fermi level. In addition to a strong thermal modulation occurring at short separations, in a region where the force is orders of magnitude larger than the one occurring at large distances, the force can be also adjusted by varying the number of graphene layers as well as their Fermi levels, allowing for relevant force amplifications which can be tuned, very rapidly and in situ, by simply applying an electric potential. Our predictions can be relevant for both Casimir experiments and micro- or nanoelectromechanical systems and in new devices for technological applications.