Generation and propagation of monochromatic acoustic phonons in gallium arsenide

We have generated coherent zone-folded longitudinal acoustic (LA) phonons by femtosecond pulse-laser excitation of a gallium arsenide/aluminum arsenide superlattice. Using the filter effect due to frequency-dependent scattering in the gallium arsenide substrate, we have determined the frequency spectrum of the LA phonons that leak out of the superlattice and propagate to a bolometer on the opposite face of the gallium arsenide substrate. We show that the phonon spectrum has a strong monochromatic component centered on ν=sLA/dSL, where dSL is the superlattice period and sLA is the phonon speed. We propose that such phonons may be used for phonon spectroscopy and in a range of “phonon optics” applications.