Abstract

We present a comparative study of time-resolved transmission and reflection measurements on thin GaAs and InP layers in the femtosecond regime. The experimental configuration is a conventional pump-probe setup using amplified CPM pulses with duration of 60 fs for excitation at 2eV and a white light continuum ranging from 1.3 to 1.9 eV as a probe. The resulting differential changes in transmission and reflection (DTS, DRS) are recorded by an optical multichannel analyzer. The differential spectra are numerically corrected^[1] for the inherent chirp of the probe continuum. The excitation density is varied from 10¹⁹ cm⁻³ down to the very low value of 10^l6cm⁻³. Figure 1 shows as an example the obtained DTS for an InP-film at different delay times after excitation. For both materials an extremely broad nonthermal distribution is observed within the first 200fs. For longer delay times, the nonequilibrium distribution is changing to a Maxwellian distribution close to the bandedge. The subsequent cooling of the thermalized distribution takes place on a picosecond time scale. Additionally, a pronounced bleaching peak emerging from probing the split-off to conduction band transtiton is clearly resolved for InP as well as for GaAs. Exciton bleaching accompanied by spectral oscillations due to coherent effects^[2] for negative delay times can be observed at the bandedge.

© 1992 IQEC