Light emission in polar group-III nitride quantum wells optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Such excitons are characterized by a non-zero dipole moment and long radiative lifetimes, because their constituent electron and hole are spatially separated by the built-in electric field in the growth direction. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

Light emission in polar group-III nitride quantum wells (QWs) optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Whereas thin GaN QWs with fast radiative lifetimes are employed in light-emitting devices, the excitons in thick QWs (typ. 7nm) are characterized by a non-zero dipole moment and long radiative lifetimes, due to the built-in electric field. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

Semiconductor coupled quantum wells (CQWs) offer an excellent laboratory for studying both intra-well direct excitons (DX) and inter-well indirect excitons (IX), as well as their interactions and spin dynamics. The presentation will focus on the pump-probe spectroscopy, a powerful tool of nonlinear optics, that we have applied to biased CQW. We will show how DX and IX spin and population dynamics, as well as the spin polarization of residual electrons may be detected via the modulation of reflectivity and Kerr rotation spectra. In particular, the origin of the nonlinearities related to the low oscillator strength IXs will be addressed. Spin relaxation and decoherence mechanisms for DX, IX and electron gas will be presented. Results on IXs created via magnetic Stark effect and the resulting spin dynamics in unbiased quantum wells will be discussed

Du fait de ses propriétés intéressantes, le domaine de fréquence térahertz (THz) du spectre électromagnétique peut avoir de nombreuses applications technologiques, de l'imagerie à la spectroscopie en passant par les télécommunications. Toutefois, les contraintes technologiques empêchant l'émission et la détection efficaces de ces ondes par des systèmes conventionnels ont valu à cette partie du spectre électromagnétique le nom de gap THz. Au cours des deux dernières décennies, plusieurs solutions novatrices sont apparues. Parmi elles, l'utilisation de transistors à effet de champ s'est imposée comme une solution originale, bon marché, avec un fort potentiel d'intégration. Le mécanisme identifié fait intervenir l'interaction entre les ondes THz et des ondes de courant (dites ondes plasma) dans le canal du transistor. Le canal du transistor agit tel une cavité pour ces ondes plasma. Le dispositif peut alors se comporter de manière résonante ou non-résonante en fonction de divers paramètres. Dans ce manuscrit, nous étudions numériquement ces différents régimes à l'aide de modèles hydrodynamiques. Les modèles utilisés élargissent les phénomènes pris en compte dans de précédentes études théoriques. Les résultats portent sur la détection d'ondes THz par des transistors et dans une moindre mesure sur leur émission. Dans le régime non-résonant, nous étudions dans quelle mesure la plage de linéarité de détection peut être étendue. Dans le régime résonant, nous montrons l'existence de nouvelles fréquences de résonance, permettant d'élargir le spectre d'intérêt de ces détecteurs.

Lumière sur le photon : des fondements de la mécanique quantique aux applications

Lasers at nanoscale : when quantum effects meet lasers

Diluted Magnetic Semiconductors (DMS) form a new class of magnetic materials, which fill the gap between ferromagnets and semiconductors. In the early literature these DMS were often named semimagnetic semiconductors, because they are midway between non magnetic and magnetic materials.DMS are semiconductor compounds (A1−xMxB) in which a fraction x of thecations is substituted by magnetic impurities , thereby introducing magnetic properties into the host semiconductor AB. This makes a great difference with semiconducting ferromagnets, i.e., ferromagnetic materials exhibitingsemiconductor-like transport properties, which have been known for some time (see a review in [2]). A DMS is expected to retain most of its classical semiconducting properties, and to offer the opportunity of a full integration into heterostructures, including heterostructures with the host material. The greatchallenge and ultimate goal of the research in this field is to obtain DMS ferromagnetic at room temperature, which can be integrated in semiconductor heterostructures for electronic or optoelectron c applications. This is one of the key issue for the advent of spintronics devices. Among the principal DMS families, II-VI and, to a less extent, III-V based DMS, with Mn as the magnetic impurity, are best understood. For this reason the present chapter will be mainly based on these compounds to introduce the well established basic physics of DMS. More details can be found in reviewpapers such as [3–5]. Some issues related to work in progress, generally on novel materials, will be also discussed but only briefly.