Generation of Surface Acoustic Waves by Rapidly Scanned Laser Beam

It has been shown that surface acoustic waves (SAW’s) were generated by a rapidly scanned laser beam. The rapid scanning generates a moving heat pulse with duration of d/V where d is the width of laser beam and V is the scanning velocity. Then SAW is generated by this heat pulse and it is coherently amplified if V is equal to its pulse velocity. To verify this principle, a long pulse YAG laser (typical pulse width of 2 ms and pulse energy of 4 J) was scanned by a rotating polygon mirror and focussed on solid surface by a cylindrical lens. On a thick aluminum block, a broad band pulse of unidirectional Rayleigh wave was observed and the amplitude was as large as that piezoelectrically generated by 200 V electric pulses. On aluminum plates of 1.2 to 2.0 mm thickness, many cycles of oscillation were observed. The frequency F of the oscillation varied as a function of the scanning velocity V and the relation between V and F agreed with the phase velocity dispersion curves of fundamental symmetric and asymmetric Lamb waves. The advantages over other laser-based methods: (1) Preventing the laser ablation damages by virtue of the low peak power. (2) Unidirectional beam propagating in only one direction (3). Selecting single mode acoustic waves by specifying the phase velocity.