Professor Robert Taylor

Abstract:

We present time-, temperature- and intensity-dependent photoluminescence measurements of undoped, n-type and p-type GaN epilayers. In the nominally undoped samples we observe at low temperatures the trapping of free excitons by neutral donors and subsequent radiative recombination. In the n-type Si-doped samples bound-exciton luminescence is dominant over a wide range of temperatures. The luminescence from p-type Mg-doped samples is dominated by shallow-donor-shallow-acceptor pair recombination and by a deep blue centres at 3.0 eV. These two emission bands show identical temperature and linear intensity dependence.

Abstract:

The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum welts is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theory for Rayleigh scattering from two-dimensional excitons subjected to disorder. These rise times are compared directly with T-2 times measured by time-integrated four-wave mixing (FWM). The comparison of the dynamics displayed in time-resolved secondary radiation and time-integrated FWM provide a clear understanding of the coherence properties of QW excitons in the first few picoseconds after excitation. High-contrast oscillations that are due to quantum beats between the heavy- and light-hole Is-states are seen. The visibility decay at very low densities is long tau(HL) = 25 ps and is related to the action of potential fluctuations on the scattering of heavy-hole and light-hole excitons. (C) 1998 Elsevier Science B.V. All rights reserved.

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. © 1998 The American Physical Society.

Abstract:

Time-resolved photoluminescence has been used to investigate the optical properties of a narrow 25 Angstrom Zn0.8Cd0.2Se/ZnSe multiple quantum well structure as a function of carrier density and temperature. At low temperatures, it is found that the luminescence is excitonic over all densities up to and above threshold. The underlying gain mechanism in cleaved 380 mu m cavities is excitonic in nature. In the low-density regime, the luminescence remains excitonic up to room temperature. (C) 1998 Elsevier Science B.V. All rights reserved.