Professor Robert Taylor

Abstract:

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by T-2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton-phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.

Abstract:

Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing between the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carrier-carrier interaction is responsible for the ultrafast transitions.

Abstract:

Femtosecond time resolution of resonantly excited secondary emission was achieved by frequency upconversion in a two-color scheme for the detection of the sum frequency signal. The quadratic rise time was determined by the inverse of the absorption linewidth. The temporal shape of the emission was independent of the emission angle, demonstrating that exciton localization lengths does not exceed the wavelength of light. The hh/lh exciton beats were related to a correlation of the in-plane center of mass potentials Hhh(R) and Vlh(R) acting on either type of excitons.

Abstract:

Using luminescence upconversion with 100 fs resolution, we have investigated the intersubband scattering of electrons in GaAs quantum wells (QWs). The energy separations between the first and second confined electron subband (E12) were more or less than the LO phonon energy (ELO). In narrow QWs (E12 > ELO) the intrinsic LO phonon scattering rate is found to be 1.6 × 1012 s-1. A Monte Carlo simulation allows us to reproduce the experimental data with high accuracy. In wide QWs (E12 < ELO) we find a very fast thermalization of the carrier population in the QW within times shorter than 300 fs. The intersubband relaxation rate for electrons in the n = 2 subband can be estimated to be shorter than 1.5 ps. © 1997 Academic Press Limited.