Focusing of large-wave-vector phonons in GaAs

Focusing of 0- to 1.5-THz transverse-acoustic phonons in GaAs is studied on the basis of an extended rigid-ion model, which reproduces very accurately the observed dispersion curves of acoustic phonons. The shifts of phonon caustics due to lattice dispersion become conspicuous at frequencies above 1 THz. Calculated focusing patterns of ballistic phonons are found to be in good agreement with the characteristic features of phonon images obtained recently by Wolfe and Northrop. We also compare the dispersive shifts of caustics in GaAs with those in Ge and discuss its implication to the phonon-focusing experiments with large-wave-vector phonons