Optical absorption effect on laser-generated resonances in semi-transparent solid

Abstract

We studied the effect of optical absorption on the Lamb wave thermoelastic generation in isotropic plates. For metallic plates, high optical absorption results in a surface source and Lamb mode amplitudes depend on the total deposited energy and laser source shape. In the case of moderately absorbing plates, the laser beam can penetrate the sample to some optical depth, producing a bulk source. Consequently, the radiation characteristics of laser-ultrasound are significantly different. The displacement amplitudes can be controlled by the incidence angle of the laser beam and depend on the material absorption. We used a laser-based ultrasonic setup for the generation and detection of guided waves in semi-transparent plate-like structure. In a free elastic plate, resonances occur at Zero-Group Velocity (ZGV) points where the phase velocity remains finite. For studying the transition from a surface source to a bulk source we performed local measurements in neutral density filters having different absorbance. ZGV resonances amplitude increase with absorption until a maximum is reached depending on the order of the mode. Furthermore, these measurements allow to discriminate the so-called thickness resonances, associated to the precursors, from ZGV resonances. Finally, the experimental results are compared with semi-analytical simulations based on Spicer model.