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, similar to 650 fs, being due to the subsequent localisation of electrons.

Abstract:

We present results of time-resolved luminescence experiments performed on Zn0.80Cd0.20Se/ZnSe quantum well laser structures as a function of carrier density and temperature. It is found that in narrow wells, where quantum confinement effects are strong, the luminescence is excitonic over all temperatures at densities up to and above threshold. For wide wells the luminescence is observed to cross over to a bimolecular decay profile at high temperature and density, which corresponds to the formation of a correlated electron-hole plasma.

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 × 10-3 S-1 cm2), and a dramatic reduction in the lifetime is observed. Stimulated emission occurs at an energy ∼ 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:

The time evolution of the stimulated emission in ZnSe/Zn(S,Se) superlattices has been studied by picosecond photoluminescence spectroscopy. The threshold of the stimulated emission is characterised by a sharp decrease of the transition lifetime from 150 ps to less than 30 ps at a carrier density of about 1012 cm-2. While the experimental results are consistent with a stimulated emission process due to excitons, they show no evidence of electron-hole plasma recombination.

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.