  We demonstrate semiconductor superlattices or nanocavities as narrow band
acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics
experiments in the transmission geometry with pump and probe incident on
opposite sides of the thick substrate, phonon generation and detection
processes are fully decoupled. Generating with the semiconductor device and
probing on the metal, we show that both superlattices and nanocavities generate
spectrally narrow wavepackets of coherent phonons with frequencies in the
vicinity of the zone center and time durations in the nanosecond range,
qualitatively different from picosecond broadband pulses usually involved in
picosecond acoustics with metal generators. Generating in the metal and probing
on the nanoacoustic device, we furthermore evidence that both nanostructured
semiconductor devices may be used as very sensitive and spectrally selective
detectors.
