Laser Excitation of Microwave Sound

Abstract

Ruby lasers can be used for exciting microwave sound in the bulk or from the surface of a crystal, by the mixing of two longitudinal modes separated by a frequency Ω in the gigacycle region. Different mechanisms can be involved; electrostrictive mixing occurs in the bulk, as mentioned by D. E. Caddes (thesis, Stanford University). Pressure of radiation and electrostriction are involved for surface excitation, with lower efficiency. Surface heating does occur even in the case of uncoated dielectrics, being related to surface damage. Our experiments were performed in lithium niobate crystals, where birefringence can be used for forward Brillouin scattering: at 810 MHz, with longitudinal sound in an X-cut crystal; at 480 MHz, with shear sound in a Y-cut crystal. We generated about 0.5 mW/mm2 of sound in the bulk of the crystal. We could not detect pressure of radiation effects, which were rapidly overcome by surface heating effects, producing up to 2 mW/mm2 of longitudinal sound in 20-nsec pulses.