Professor Robert Taylor

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.

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.