Monitoring stress power during high-power pulsed laser-material interactions

High-power pulsed laser-material interactions are monitored with acoustic measurements of mechanical stress power. At laser intensities above 10⁸ W/cm², an explosive, plasma forming interaction occurs between the laser beam and the solid target, ablating material away. Stresses in the target caused by the laser ablation can be very large, which for short pulsed laser ablation can make mechanical transport of energy significant. Experimental work is conducted to study the affect of laser intensity on stress power in the target and that carried by pressure waves in the surrounding gas medium. 30 ns excimer and 35 ps Nd:YAG lasers are used to ablate Al and Cu targets, and the resulting stresses are recorded by different types of transducers. The nanosecond results show that gas ionization breakdown can be detected through the stress power, and that the power measured follows a near quadratic dependence on the incident energy. Variations in the ratio of the shock power in air to the stress power in the target shows that plasma shielding of the laser beam can be monitored using the mechanical stress power. Differences in plasma shielding during pico-second ablation of copper in Ar versus He are shown using stress power monitoring.