Professor Robert Taylor

Abstract:

Non-thermal carrier states at early times are studied using femtosecond pump-probe spectroscopy in GaN. After the residual chirp on the continuum probe is removed, the normalized difference spectra (NDS) for different probe energies are synchronized, recovering the full time resolution of our laser pulse (120 fs). Our Monte-Carlo simulation agree well with the unchirped NDS spectrum, which shows the development of the carrier distribution at early times, where phonon satellites are seen. together with a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electron-LO phonon interaction. Employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, a measure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons. Our Monte-Carlo model agrees well with this data, and confirms the dominance of hot phonon effects at early times. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

A new way of analysing the data in a variable stripe length method gain experiment is presented. The stripe length dependence of the gain is measured in In0.02Ga0.98N/In0.16Ga0.84N multiple quantum wells (MQWs). We confirm that this arises from the change of the chemical potential along the excited stripe due to the interaction of the carrier and photon densities, and the gain threshold density is estimated. Comparison with the PL and PLE spectra suggests that the optical gain arises from weakly localised states in the quantum well in our low-indium-content sample. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

A way of analyzing the data in a variable stripe length method gain experiment is presented. We confirm that the stripe length dependence of the gain in In0.02Ga0.98N/In0.16Ga0.84N multiple quantum wells is caused by the change of the chemical potential along the excited stripe due to the interaction of the carrier and photon densities, and the gain threshold density is estimated. A trial function assuming a Lorentzian line shape for the stripe length dependence of the gain is compared with the edge emission intensity as a function of the stripe length. This is found to fit very well with our data, even beyond the saturation region. © 2001 American Institute of Physics.

Abstract:

The polarization dependence of biexcitonic signals and quantum beats between A-excitons (XA) and A-biexcitons (XAXA) in a high-quality GaN epilayer is measured by spectrally-resolved and time-integrated four-wave mixing. With cross-linear polarised light, mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between XA and XAXA, and agrees well with the calculated XAXA binding energy; while the second beating period corresponds to that between XA and donor bound excitons (D⁰X). The temperature-dependent homogeneous linewidth shows that the D⁰X has a larger acoustic phonon coupling coefficient than the XAXA. We also measured the polarization dependent B-biexciton (XBXB) signal. The effective masses for the A- and B-hole were deduced from the binding energy.

Abstract:

The polarization dependence of biexcitonic signals and quantum beats between A-excitons (X-A) and A-biexcitons (XAXA) in a high-quality GaN epilayer is measured by spectrally-resolved and time-integrated four-wave mixing. With cross-linear polarised light, mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between X-A and XAXA, and agrees well with the calculated XAXA binding energy, while the second beating period corresponds to that between X-A and donor bound excitons ((DX)-X-0). The temperature-dependent homogeneous linewidth shows that the (DX)-X-0 has a larger acoustic phonon coupling coefficient than the XAXA. We also measured the polarization dependent B-biexciton (XBXB) signal. The effective masses for the A- and B-hole were deduced from the binding energy.