Terahertz phonon optics in GaAs/AlAs superlattice structures

We have generated pulsed beams of ~THz monochromatic longitudinal acoustic phonons by ultrafast laser excitation of GaAs/AlAs superlattices. The phonons propagated ballistically across the GaAs substrates at low temperatures and were detected using superconducting aluminum bolometers. Between the generator superlattice and the bolometer was a second superlattice structure which acted as a reflective band-stop filter. We have made measurements of the phonon pulses in various samples for which the filter stop band was either in tune or out of tune with the generator frequency. These have enabled us to place an upper bound on the bandwidth of the monochromatic phonon beam. We have also determined the relative proportions of the detected longitudinal acoustic phonon signal that are due to monochromatic phonons and broadband phonons from carrier relaxation. We present this experiment as an example of ``phonon optics'' using propagating monochromatic phonons in the THz frequency range.