Subpicosecond time-resolved reflection of ultrafast electrical pulses from GaAs in the presence of nonthermal photoexcited carriers

The reflected waveform of a femtosecond electrical pulse incident on GaAs following hot-electron excitation by a subpicosecond optical pulse is calculated. Conditions under which the electrons initially relax mainly by emitting a cascade of longitudinal-optical (LO) phonons are assumed to hold. It is found that the electrical pulse can show amplification on reflection when the initial kinetic energy of the excited electrons equals an integral multiple of the LO-phonon energy within a sufficiently narrow energy spread.