Folded acoustic phonons in GaAs/AlAs corrugated superlattices grown along the [311] direction

We present Raman spectra for folded acoustic phonons in GaAs/AlAs superlattices grown along the [311] direction. The symmetry of this direction offers the rare possibility of observing folded phonons of all three acoustic branches. In accordance with the predicted selection rules the Raman spectra show doublets of modes of pure transverse (polarized along the [01\ifmmode\bar\else\textasciimacron\fi{}1] direction) as well as mixed transverse and longitudinal character. The frequencies and strengths of these modes are also in agreement with calculations based on the continuum model. Folded-phonon to Brillouin ratios are calculated using elasto-optic constants of GaP in lieu of AlAs.