Coupled Brillouin-Raman study of direct and folded acoustic modes in GaAs-AlAs superlattices

Acoustic and acousto-optic properties are investigated in GaAs-AlAs superlattices of period D ranging between 40 and 500 Å. An experimental setup where a Fabry-Perot interferometer is used in tandem with the Raman spectrometer allowed frequency-shift measurements as low as 1 ${\mathrm{cm}}^{\mathrm{-}1}$. The dispersion branches of (direct) LA and TA, as well as folded-LA and -TA, modes are obtained for excitation wave vectors q less than the limit π/D of the first Brillouin zone, and in the range π/D to 2π/D (second Brillouin zone). The intensities of all these modes are very sensitive to the reduced excitation wave vector Q=qD/π. Many striking effects are pointed out for the first time: The existence of folded acoustic branches which are observed at frequencies below the direct acoustic branch for Q>1, and the ‘‘anomalous’’ intensity behavior, certain modes becoming even more intense than the Brillouin line for Q approaching the Brillouin-zone limit Q=1. Determination of the relative value p of the Pockels photoelastic coefficient $p_{13}$ in AlAs with respect to GaAs is also reported. p is observed to undergo a noticeable variation versus the photon energy between 6764 and 4579 Å.