Influence of the absorbed optical quanta energy on GHz ultrasound generation in GaAs

Experimental results of the investigation of the optoacoustic processes taetang place in GaAs semiconductor at ultrashort time scales are reported. Femtosecond laser has been used both for the generation (through the deformation potential mechanism by the interband absorption of laser radiation) and detection of GHz ultrasound waves. First experimental observation of an abrupt change in the phase of the photoexcited GHz ultrasound, when with increase of the energy of optical quanta direct generation of the electron-hole pairs in the side-valleys of GaAs becomes allowed by the energy and momentum conservation laws, is reported. We relate this observation, at least partially, to abrupt change in the ultrafast dynamics of photogenerated electron-hole plasma, in particular to deceleration of plasma diffusion when heavy carriers in the high energy side valley are photogenerated instead of light carriers in the lowest energy valley.