Professor Robert Taylor

Abstract:

We report the observation of an ultrafast optical nonlinearity in ZnSe/ZnSeS superlattices which arises from the generation of an internal field due to charge separation caused by preferential hole trapping. The absorption recovery is extremely fast, ∼ 650 fs, being due to the subsequent localisation of electrons.

Abstract:

Time-resolved luminescence has been used to investigate the dynamics of excitonic recombination and stimulated emission in ZnCdSe/ZnSe multiple quantum wells where the exciton binding energy is close to the optical phonon energy. At low excitation densities strong excitonic recombination is observed with simple exponential decay kinetics. At densities near threshold, however, bilinear recombination kinetics are found with a large rate constant (R = 1.8 x 10(-3) s(-1) cm(2)), and a dramatic reduction in the lifetime is observed. Stimulated emission occurs at an energy similar to 18 meV below the exciton absorption peak, and shifts to lower energy with increasing excitation density up to three times the threshold density. These observations are consistent with recombination in a dense exciton system, which gives way at high density to a correlated electron-hole plasma.

Abstract:

Time-resolved photoluminescence experiments in a wide carrier density range up to and beyond the threshold for stimulated emission in ZnSe/ZnSxSe1-x. superlattices are presented. Different localisation mechanisms of free excitons are identified giving rise to different radiative recombination channels. At high carrier density localised energy states merge into miniband that allows for exciton-exciton interaction, such as screening of the free-exciton absorption and inelastic exciton-exciton scattering.

Abstract:

We present a combined experimental and theoretical investigation of intra- and intersubband carrier relaxation in GaAs/AlAs quantum wells following femtosecond laser excitation at low density. Time-resolved optical absorption has been measured using 125 fs pulses from a tunable Ti:sapphire laser which allows carriers to be excited preferentially into different subbands. The experiments have been analyzed using a multi-subband Monte Carlo simulation which contains all the important scattering mechanisms. Electron-phonon scattering rates obtained from dielectric continuum theory have been used, and we find excellent agreement with the experiment. The dominance of interface phonons in intrasubband relaxation is confirmed. © 1995 American Institute of Physics.