EXCITONIC AND FREE-CARRIER QUANTUM BEATS CREATED BY FEMTOSECOND EXCITATION AT THE BAND-EDGE OF GAAS

The coherent optical response of bulk GaAs is studied after ultrafast excitation close to the band edge. Pump-probe experiments with 20-fs p ulses demonstrate that coherent polarizations between heavy- and light -hole continuum states make a strong contribution to the nonlinear opt ical response. The beat frequency is determined by the heavy-hole-ligh t-hole energy splitting in the spectral probe window. In addition, exc itonic heavy-hole-light-hole quantum beats are observed in the slightl y strained GaAs crystal. The different contributions are separated via their polarization selection rules. [S0163-1829(98)02340-6].