Generation and directivity control of bulk acoustic waves by phase velocity scanning of laser interference fringes

A novel method for generating high-frequency bulk acoustic waves (BAW) using scanning interference fringes (SIF) is presented. The SIF are produced by intersecting two coherent laser beams with different frequencies on an opaque specimen. It is possible to generate unidirectional BAW which propagate into the specimen by this method. Propagation direction of the BAW is determined by the phase-matching condition between the SIF and the wave front of the BAW at the surface, which could be controlled by incident angles of laser beams. The principle of this method and experimental verification using a sintered Si₃N₄ specimen are described. Experiments were carried out using a 15 mJ/130 ns Q-switched Nd:YAG laser for 80 MHz BAW generation and optical knife-edge method with 50 mW argon-ion CW laser for detection. Directivity of BAW was steered from 24.5° to 77.5° by changing the incident angles of the laser beams from 0.066° to 0.136°. BAW were found to be well collimated especially in the near field.