Acoustic mode dispersion at hypersonic frequencies in GaAs

We report the results of a Brillouin study, carried out in 180° backscattering using 457.9–514.5 nm radiation, of the longitudinal acoustic mode traveling along the [001] crystal axis in GaAs. The mode line width exhibits opacity broadening due to increasing optical absorption with decreasing incident light wavelength. The wave velocity of this acoustic mode is determined at hypersonic frequencies from the measured phonon dispersion in GaAs. The Brillouin data for the velocity and its associated elastic modulus c11 are compared with values determined from lower-frequency ultrasonic and other measurements. As is the case for Si and Ge [M.H. Kuok, S.C. Ng, Z.L. Rang, D.J. Lockwood, unpublished], the measured hypersonic velocity in GaAs is found to be ≈1% lower than that of the ultrasonic, but the different measurements generally agree within experimental accuracy.