A New Method for Calculation of Laser-Generated Ultrasound Pulses

Abstract

The generation of acoustic pulses (in solids) by laser pulses has received considerable attention recently (an extensive review has been given by Hutchins[l]). Current applications are to nondestructive evaluation and materials characterization, where it is convenient to have a highly reproducible source requiring no contact with the sample [2–5]. The need to make these applications quantitative requires a theoretical model which:1) is based on fundamental principles; 2) allows the use of realistic sample and source properties; and 3) is readily usable by the research community without a major computational development effort. Doyle[6] and Schliechert et al.[7] have described approaches which meet the first two criteria, but which are very computation-intensive. We will describe and illustrate a new formulation[8] which meets all three criteria. Numerical calculations will be presented to illustrate the efficacy of this approach, with emphasis on the effects of finite source dimensions and sample surface modification. Comparison with previous point-source results will indicate when the latter may he used. Finally, we show that the small initial displacement “spike” observed in experiments with metal samples, is due to “mode conversion”(thermal-to-longitudinal) at the boundary, rather than to the finite size of the thermal source resulting from thermal diffusion. For the present we limit the discussion to the thermoelastic regime.