Subpicosecond generation and decay of coherent phonons in III-V compounds

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