Terahertz Response of Acoustically-Driven Optical Phonons

The manipulation of TO-phonon polaritons and the terahertz (THz) light field associated with them by means of an ultra-sound acoustic wave is proposed and illustrated by calculating the TO-phonon-mediated THz response of acoustically-pumped CuCl and TlCl crystals. We show the high-contrast acoustically-induced change of the THz reflectivity alongside with multiple THz Bragg replicas, which are associated with the infrared-active TO-phonon resonance driven by the ultrasonic wave. The effect, which stems from phonon anharmonicity, refers to an operating acoustic intensity I_ac ~ 1-100 kW/cm^2 and frequency nu_ac ~ 0.1 - 1 GHz, with possible applications in THz spectroscopy.