Observation of coherent zone-folded acoustic phonons generated by Raman scattering in a superlattice

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser exci tation wavelengths shorter than 767 nm (the energy of E1HH1 transition), we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been o bserved in this structure. We suggest that the excitation wavelength and an gular characteristics of the longitudinal acoustic emission is consistent w ith those of propagating modes produced as coherent phonons "leak" from the superlattice structure. (C) 2000 American Institute of Physics. [S0003- 69 51(00)00246-1].