Laser generated ultrasound: a thermoelastic analysis of the source

Abstract

Abstract The purpose of this work is to study the behavior of laser ultrasound sources in isotropic metals using ultra-short laser pulses in the thermoelastic regime. Temporally Gaussian and high frequency modulated laser pulses are investigated, and numerical results show that the thermal response rates of both steel and aluminum are much faster than their mechanical response rates. This indicates that the temporal point source limit in laser ultrasonics is a mechanical rather than thermal limitation. Two temporal point source limits are identified. For waves generated by the SCOE (surface center of expansion), it is shown that the temporal point source limit occurs at a Gaussian rise time of 1 ns in both aluminum and steel. However, the precursor is a result of thermal diffusion and does not display a temporal point source limit within the range of parameters considered here. A dimensional analysis of the laser source is conducted, and it is found that a single dimensionless parameter defines thermal similarity for laser sources, but no single parameter can define acoustic similarity. The physical dimensions of the laser source are estimated for steel and aluminum, and it is found that for temporally and spatially Gaussian laser pulses the outer edge of the source is 1.65 times the Gaussian spot radius over a wide range of rise times and spot sizes. Also, a dimensionless parameter is given for three dimensional problems which predicts minimum rise times and maximum modulation frequencies for which hyperbolic heat conduction effects will be of no significance in the ultrasonic displacements.