Imaging of acoustic phonon stop bands in superlattices

Phonon-imaging experiments reveal the angular distribution of high-frequency acoustic-phonon transmission through an ${\mathrm{In}}_{0.15}$ ${\mathrm{Ga}}_{0.85}$As/AlAs superlattice. Phonon stop bands due to Bragg reflection at the folded-zone boundary are observed for both longitudinal and fast transverse phonons. A newly predicted intrazone stop band due to coupling between acoustic modes is detected.