Generation of nonequilibrium optical phonons in GaAs and their application in studying intervalley electron-phonon scattering

We present a detailed theoretical and experimental investigation of the generation and detection of nonequilibrium longitudinal-optical (LO) phonons in GaAs by picosecond laser pulses. The nonequilibrium LO phonons are excited by the relaxation of energetic electron and hole pairs created by a tunable and mode-locked dye laser. The photoexcited LO-phonon population, monitored by Raman scattering with the same excitation laser pulses, is measured as a function of the excitation photon energy keeping the laser power constant. The phonon population was found to increase monotonically with photon energy between 1.75 and 1.95 eV. Above 1.95 eV the phonon population became roughly constant until at 2.105 eV the phonon population suddenly decreased by approximately a factor of 2. The experimental results are compared with several theoretical model calculations. We found that a simple parabolic bands model failed to explain the experimental results. Only by using a realistic band structure for GaAs did we achieve quantitative agreement between theory and experiment. In particular, the experimental results turned out to be very sensitive to details of the band structure of GaAs such as nonparabolicity of the ${\Gamma }_1$ conduction band, warping of the valence bands, and presence of higher-conduction-band minima at zone boundaries. Specifically, the sudden decrease in the LO-phonon population at 2.105 eV is attributed to transfer of energetic electrons from the ${\Gamma }_1$ valley to the higher-energy minima near the $X$ points. By analyzing this decrease, we were able to determine precisely the values of intervalley electron-phonon deformation potentials.