Les excitons indirects dipolaires, sont des quasi-particules bosoniques dans les semi-conducteurs, composés d'un électron et d'un trou spatialement séparés mais toujours liés par interaction coulombienne. Leur grande durée de vie radiative et leur capacité à se déplacer sur de grandes distances avant leur recombinaison en font un système unique qui peut être à la fois optiquement actif mais également électriquement contrôlable. Ce système permet l'étude des propriétés fondamentales de la lumière et de la matière, mais aussi le développement de dispositifs excitoniques conceptuellement nouveaux. Les excitons dans les puits quantiques polaires en nitrure de gallium (GaN) peuvent être considérés comme des excitons naturellement indirects, en raison du fort champ électrique intrinsèque existant dans la direction de croissance cristalline. Cette thèse est consacrée à l'étude expérimentale et une étude des excitons indirects dans des hétérostructures GaN/(Al,Ga)N, ainsi que les états collectifs, depuis la conception et la fabrication jusqu'à la spectroscopie optique de leurs états collectifs.Les principaux résultats de ce travail sont (i) la démonstration du confinement spatial dans le plan et du refroidissement des excitons indirects, lorsqu'ils sont piégés dans le potentiel électrostatique créé par des électrodes semi-transparentes de géométries diverses soigneusement conçues et déposées sur la surface de l'échantillon, qui est une condition préalable à l'étude du diagramme de phase complexe de ces bosons dipolaires à basse température ; (ii) la preuve de principe du contrôle électrique des densités et des flux d'excitons indirects dans le plan du puits quantique. Cela ouvre des perspectives intéressantes pour la réalisation de dispositifs excitoniques ; (iii) les premiers points sur le diagramme de phase des bosons dipolaires, fournissant une première mise en évidence non seulement de l'existence d'un phase fortement corrélée résultant des corrélations induites à forte densité (phase de liquide dipolaire) mais aussi la dissociation (transition de Mott) de ces excitons indirects dans les hétérostructures GaN/(Al,Ga)N.

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.

We investigate the demonstration and quantification of the strong coupling between excitons and guided photons in a GaN slab waveguide. The dispersions of waveguide polaritons are measured from T = 6 to 300 K through gratings. They are carefully analyzed within four models based on different assumptions, in order to assess the strong-coupling regime. We prove that the guided photons and excitons are strongly coupled at all investigated temperatures, with a small (11%) dependence on the temperature. However, the values of the Rabi splitting strongly vary among the four models: the "coupled oscillator" model overestimates the coupling; the analytical "Elliott-Tanguy" model precisely describes the dielectric susceptibility of GaN near the excitonic transition, leading to a Rabi splitting equal to 82 ± 10 meV for fundamental transverse-electric mode; the experimental ellipsometry-based model leads to smaller values of 55 ± 6 meV. We evidence that, for waveguides including active layers with large oscillator strengths, as required for room-temperature polaritonic devices, a strong bending of the modes' dispersion is not necessarily the signature of the strong coupling, which requires for its reliable assessment a precise analysis of the material dielectric susceptibility.

Low-threshold lasing under pulsed optical pumping is demonstrated at room temperature for III-nitride microdisks with InGaN/GaN quantum wells on Si in the blue spectral range. Thresholds in the range of 18 kW/cm 2 have been achieved along with narrow linewidths of 0.07 nm and a large peak to background dynamic of 300. We compare this threshold range with the one that can be calculated using a rate equation model. We show that thresholds in the few kW/cm 2 range constitute the best that can be achieved with III-nitride quantum wells at room temperature. The sensitivity of lasing on the fabrication process is also discussed.

Ultraviolet microdisk lasers are integrated monolithi-cally into photonic circuits using a III-nitride on silicon platform with gallium nitride (GaN) as the main waveguiding layer. The photonic circuits consist of a microdisk and a pulley waveguide terminated by out-coupling gratings. We measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 nm to 399 nm under pulsed excitation, achieving low threshold energies of 0.14 mJ/cm 2 per pulse (threshold peak powers of 35 kW/cm 2). A large peak to background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.