Résumé
GaN/AlN quantum dots are semiconductor nanostructures where the huge conduction band offset of 1.75 eV leads to intraband transitions at exceptionnaly short wavelengths compared to III-As semiconductor materials. For specific quantum dot dimensions, it was shown that the absorption spectrum covers the optical communication spectral range. The ultrafast relaxation dynamics observed for these materials motivates the development of opto-electronic devices, and especially ultrafast switches. However, a detailed insight into the microscopic processes governing the relaxation dynamics is still missing although of fundamental importance for the understanding and the optimization of the devices operation. In particular, there is no study of the homogeneous broadening which drives the saturation properties of GaN/AlN quantum dots-devices. In this paper, we present homogeneous linewidth measurements by means of the nonlinear optical technique of spectral hole-burning. Spectrally-resolved pump-probe experiments with cw-lasers allow us to characterize the coherence relaxation dynamics down to 5K by resolving the homogeneous linewidth in an ensemble of GaN/AlN quantum dots. The differential transmission measured at 5K for a TM-polarized pump shows a pronounced bleaching with an amplitude of 2x10-4 on a background signal of 5x10-5. According to the selection rules of the s-pz intraband transition in GaN/AlN QDs, we attribute the photoinduced increase of the probe transmission to the partial saturation of the quantum dots by the pump laser, and we deduce a homogeneous linewidth of 15 meV at 5K. We find that the differential transmission signal scales like the square-root of the incident pump power, thus revealing the importance of electron-electron scattering in the population relaxation dynamics. As a matter of fact, we find that the population relaxation from the pz excited state is predominantly governed by Auger-type processes whereas optical phonon emission only plays a minor role, even if it may contribute to secondary processes during the carrier cascade to the fundamental s state via intermediate confined states.