Laser-based ultrasound measurements of optical absorption depth in epoxy resins

Pulsed lasers can generate ultrasound from stresses due to rapid thermal expansion. In this low power thermoelastic regime the material is not damaged. This paper concentrates on epoxy resins and aims to relate the observed amplitude of the longitudinal wave to the optical absorption depth of the epoxy. The ultrasound is generated using a high power pulsed laser and the absolute amplitude of the ultrasound is measured with a Michelson interferometer. In the thermoelastic regime, the laser beam is focused onto the sample, causing rapid expansion in times that are comparable to the rise time of the laser pulse. In metals, the laser radiation is absorbed in the thin electromagnetic skin depth but in non-metals the phenomenon is dominated by the optical absorption depth. The latter can vary from a few microns to several millimeters for materials such as epoxy resins. As a consequence, a bigger volume of the material is affected. the temperature rise is less and the amplitude of the longitudinal wave is greater. This condition is referred to as "a buried thermoelastic source". Two lasers were used in this study: a TEA CO₂ and a XeCl excimer laser. The results are compared with optical transmission measurements.