Observation of coherent folded acoustic phonons propagating in a GaAs/AlAs superlattice by two-color pump-probe spectroscopy

The coherent folded longitudinal acoustic (FLA) phonons in a GaAs/AlAs superlattice have been studied using a two-color pump-probe technique and a designed sample in which the superlattice layer is sandwiched by AlₓGa_1-xAs cladding layers. The two-color pump-probe technique makes it possible to separate regions of generation and detection of coherent phonons, which provides a powerful tool to examine the spatial dynamics of coherent phonons. We have found two types of coherent FLA phonons: the first-order FLA phonons with finite wave vectors (q≠0) and the zone-center mode of the first-order FLA phonons $(q=0).$ When ballistic acoustic phonons are generated near the surface region of the AlₓGa_1-xAs cladding layer and propagate through the superlattice layer, the first-order FLA phonons with q≠0 are observed in the superlattice layer. On the other hand, when the FLA phonons are generated within the superlattice layer, the zone-center FLA phonon extending from the superlattice layer is detected near the surface region of the AlₓGa_1-xAs cladding layer.