Terahertz acoustic oscillations by stimulated phonon emission in an optically pumped superlattice

A source of nanometer wavelength, transverse-polarized, sound waves based on stimulated emission of phonons in a semiconductor superlattice inside an acoustic cavity and pumped by a low-average power nanosecond pulsed laser is described. The experimental evidence for sound laser (saser) action in our device is a superlinear increase in emitted acoustic intensity and an increase in emission directionality above a threshold in the peak optical pumping power at $\ensuremath{\sim}500\text{ }\text{W}\text{ }{\text{cm}}^{\ensuremath{-}2}$. We also show that saser action is theoretically possible in the device due to strong electron-phonon coupling, the achievement of population inversion under optical pumping and effective phonon confinement in an acoustic cavity.