Observation of subpicosecond transient driving force for coherent phonons in GaAs by double-pulse excitation

Temporal variation in the driving force of coherent phonon was studied in n-type GaAs by a double-pulse pump–probe experiment by changing the pulse separation Δt. Coherent phonons induced by the second pulse corresponding to the LO-phonon mode and the LO-phonon–plasmon coupled mode rapidly decreased in amplitude to their minima in ∼5 ps, then slowly recovered. This attenuation was explained by a screening of the surface space–charge field due to photo-excited carriers, while the recovery was attributed to a weakening of the screening through recombination and diffusion of photo-excited carriers.