Acoustic field of focused bulk waves generated by phase velocity scanning of laser interference fringes

摘要

In conventional methods of laser ultrasound, the efficiency, mode - selectivity and directionality are insufficient for precise measurements. To overcome these difficulties, phase velocity scanning (PVS) was proposed, and a single - mode, high frequency unidirectional ultrasound was efficiently generated by the scanning interference fringes (SIF) approach. In this paper, as a novel extension of the SIF approach, a converging SIF (C - SIF) is proposed to focus a bulk acoustic wave (BAW) with a large amplitude. C - SIF is formed by intersecting a focused laser beam and a collimated beam with different frequencies on a specimen, and consists of concentric circles scanned into the center with scanning velocities faster for inner circles. Focusing of a 110 MHz BAW is verified using second harmonic waves of a Q - switched Nd:YAG laser and an Al specimen. It was confirmed that the focal point of the focused BAW was controllable by changing the defocus length of the focused laser beam on a specimen.