Abstract
Sound amplification in an electrically biased superlattice (SL) is studied in optical experiments with 100fs time resolution. Coherent SL phonons with frequencies of 40, 375, and 410GHz give rise to oscillatory reflectivity changes. With currents from 0.5 to 1.3A, the Fourier amplitude of the 410GHz phonon increases by more than a factor of 2 over a 200ps period. This amplification is due to stimulated Čerenkov phonon emission by electrons undergoing intraminiband transport. The gain coefficient of 8×10^{3} cm^{-1} is reproduced by theoretical calculations and holds potential for novel sub-THz phonon emitters.
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