Introduction . Sound waves are important as high-frequency signal carriers and as a means to distort crystal lattices in space and time. Due to the slow speed of sound compared to light, ultrashort sound wavelengths in the nanometer scale are attainable in the terahertz (THz) domain, the highest sound frequencies in solids. The control and generation of THz sound waves offers a route to nanoscale ultrasonic imaging, the generation of THz electromagnetic radiation1 , 2 , 3, and ...