Professor Robert Taylor

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.

Abstract:

This paper shows experimental data and simulation results of the gain recovery of gain-clamped semiconductor optical amplifiers (GC-SOA). Damped relaxation oscillations are observed. Both the oscillation frequency and damping rate increase with applied current and can attain very high values. We measured a gain recovery time of 26 ps at 200 mA applied current and an oscillation frequency of 11 GHz. A good agreement between experimental data and simulation results is obtained. Simulation results indicate that a GC-SOA used as amplifier in wavelength division multiplexing (WDM) networks can amplify signals at very high bit-rates with low cross-talk and extinction ratio compression.

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 T2/2. 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:

We study the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radiation. At high exciton densities, the emission is due to incoherent luminescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger than their diameter, the temporal shape is independent of density and rises quadratically, in excellent agreement with recent theories for resonant Rayleigh scattering. © 1997 The American Physical Society.