Optodynamic analysis of nonlinear acoustic waves induced by pulsed laser ablation

We examine waveforms (time histories) of nonlinear spherical acoustic waves generated during pulsed Nd:YAG laser ablation of solid absorbing targets in air. The waveforms are detected by means of a thin HeNe probe beam located at a distance of few millimeters above the ablated spot on the target surface. Angular deflection of the probe beam due to the refractive index gradients induced by the passage of the acoustic wave is converted to signal voltage by a quadrant photodetector. We find that the acoustic wavefront transit times decrease with the increase of ablation laser energy and take that as a clear evidence that a shock is formed on the wavefront. We investigate the nonlinear evolution of the waveforms in time and frequency domain and employ the weak shock solution of the point explosion model to explain the observed features.