Professor Robert Taylor

Abstract:

We present optical gain and loss spectra measured over a range of carrier densities at low temperature in hexagonal GaN epilayers. We have determined the optical loss directly to be similar to 80 cm(-1). Photoluminescence spectra show that stimulated emission in our samples arises from electron-hole plasma recombination. Time-resolved pump-probe transmission experiments have shown that there is considerable bandgap renormalization for carrier densities at which stimulated emission occurs.

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.