Abstract
In conventional methods of laser ultrasound, the efficiency, the mode selectivity and the directionality were insufficient for precise measurements. In order to overcome these difficulties, the phase velocity scanning (PVS) method [K. Yamanaka et al., Appl. Phys. Lett. 58, 1591 (1991)] of the scanning interference fringe (SIF) approach was proposed, and a single-mode, high-frequency unidirectional ultrasound was efficiently generated [H. Nishino et al., Appl. Phys. Lett. 62, 2036 (1993)]. But in this approach, the amplitude of the bulk acoustic wave (BAW) was not as large as that of the surface acoustic wave, because the interaction length between the fringe and the generated ultrasounds was shorter (Nishino et al., 1993). In this paper, as a novel extension of the SIF approach, a point focusing type of SIF (PFT-SIF) is proposed to focus the BAW with a large amplitude. By intersecting a focused laser beam and a collimated beam with different frequencies on a specimen, a PFT-SIF is formed, which consists of concentric circles scanned into the center with scanning velocities faster for inner circles. Verification is carried out using second-harmonic waves of a Q-switched Nd:YAG laser and an Al specimen, and profiles of focused acoustic fields for 50- to 80-MHz BAWs are studied.
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