Evidence for the existence of guided longitudinal acoustic phonons in ZnSe films on GaAs

We report theoretical as well as experimental evidence for a new class of long-wavelength acoustic film excitations which we identify as longitudinal guided acoustic phonons. Despite a coupling to propagating transverse film modes, these new modes have lifetimes comparable to that of the Rayleigh mode and are the longitudinal counterpart to the well-known Lamb or Sezawa modes, with analogous existence criteria. The properties of the new modes are discussed in detail for ZnSe films on GaAs, the material system for which we were able to observe these modes by Brillouin spectroscopy.