Acoustic mode dispersion at hypersonic frequencies in GaAs

We report the results of a Brillouin study, carried out in 180 degrees back scattering using 457.9-514.5 nm radiation, of the longitudinal acoustic mod e traveling along the [001] crystal axis in GaAs. The mode line width exhib its opacity broadening due to increasing optical absorption with decreasing incident light wavelength. The wave velocity of this acoustic mode is dete rmined at hypersonic frequencies from the measured phonon dispersion in GaA s. The Brillouin data for the velocity and its associated elastic modulus c ll 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 As is the case for Si and Ge [M.H. Kuok, S.C. Ng, Z,L. Rang, D.J. Lockwood, unpublished], the measured hypersonic velocity in GaAs is found to be approximate to 1% lower than that of the ultrasonic, but th e different measurements generally agree within experimental accuracy. (C) 2000 Elsevier Science Ltd. All rights reserved.