SUBPICOSECOND GENERATION AND DECAY OF COHERENT PHONONS IN III-V COMPOUNDS

Photoexcitation of III-V compounds with a 50 fs laser pulse leads to u ltrafast longitudinal field changes of up to 100 kV cm-1. These are ge nerated via screening and transport effects of the photoexcited carrie rs in the surface space-charge region. The field rise is sufficiently short to impulsively launch coherent LO phonons. A detailed study of t he frequency and dephasing behaviour of these phonons as a function of excitation density and excess optical excitation energy is performed in GaAs and AlGaAs.