Optoacoustic spectrum of GaP: experiment and theory

GaP is an important indirect band gap material with variety of applications in optics ranging from LEDs to possible applications in GaP/Si based solar cells. We investigated the spectral dependence of the opto-acoustic response of GaP using time-domain Brillouin scattering. We found that the amplitude of the Brillouin oscillations varies significantly near the direct transition at Γ point. The developed theoretical model quantitatively explains the experimental data and shows that one can use coherent phonon spectroscopy to provide detailed information about electronic structure and optical transitions in indirect band gap materials. The results for GaP are also compared to GaAs and Si.