Generation and detection of terahertz coherent transverse-polarized acoustic phonons by ultrafast optical excitation of GaAs∕AlAs superlattices

We have used femtosecond pump-probe techniques to generate and detect coherent transverse and quasitransverse polarized acoustic phonons in GaAs∕AlAs superlattices. Direct generation of transverse phonons is achieved using superlattices grown on the low-symmetry, (311) and (211), crystal planes. The frequency of the generated phonons is determined by the superlattice period and is in the region of 0.4THz. The dependence of the mode structure on the pump polarization suggests that a Raman scattering process is responsible for coherent phonon generation. Using bolometers on the back surface of the substrate, we show that the transverse phonons leak out of the superlattice and propagate over macroscopic distances at low temperature.