Umklapp process in observation of coherent folded longitudinal acoustic phonons in a GaAs/AlAs long-period superlattice

Coherent folded longitudinal acoustic phonons in a GaAs/AlAs long-period superlattice have been investigated by using a reflection-type two-color pump-probe technique under the condition that the wave vector of the probe pulse in the sample exceeds the mini-Brillouin zone. The coherent oscillations observed in the time-domain signals indicate the propagation of the phonon wave packet through the whole superlattice layer. The Fourier transform spectrum of the time-domain signals is compared with the dispersion relation of the folded longitudinal acoustic phonons in the long-period superlattice calculated using a transfer matrix method on the bases of an elastic continuum model. This comparison indicates that the folded longitudinal acoustic phonons in the long-period superlattice are observed through the umklapp process. PACS: 78.67.Pt, 78.47.+p, 63.22,+m