Monochromatic terahertz acoustic phonon emission from piezoelectric superlattices

We demonstrate that mismatch of the piezoelectric coefficients of the layers of a superlattice structure gives rise to the drastic enhancement of electron-phonon interaction for resonant acoustic phonons, propagating at small angle to the superlattice axis and having frequency close to 2 pi ns/d, where s is sound velocity, d is superlattice period, and n is integer. As a result, strong collimated quasi-monochromatcic beams of resonant phonons can be emitted under the relaxation of hot electrons in superlattices. This prediction is confirmed experimentally for a GaAs/AlAs superlattice photoexcited by intense femtosecond laser pulses. The spectrum of the emitted phonons is analyzed with the use of an additional filter superlattices, whose phonon stop-bands are designed to be matched or unmatched to the frequency of resonant transverse phonons of the photoexcited superlattice (in our case the latter is about 450 GHz). According to these results, a considerable portion of the phonon flux is formed by quasi-monochromatic resonant phonons. In addition, we show that the matched-filter design provides phonon-laser (saser) action under the less intense, nanosecond-pulse excitation. We attribute this to the origination of the electron inversion in the ssystem and phonon Fabri-Perot cavity formation due to the resonant phonon reflection by the filter superlattice and the top surface of the sample.