Noncontact monitoring of laser ablation using a miniature piezoelectric probe to detect photoacoustic pulses in air

We show for the first time the use of pulsed photoacoustic detection using a miniature piezoelectric transducer to study and monitor photoablation in air by an ultraviolet KrF excimer laser. At incident laser fluences below the ablation threshold, the photoacoustic signal is produced by the thermal expansion of the air near the irradiated surface ("thermal-piston signal") and propagates at the speed of sound. Above the ablation threshold, the photoacoustic signal contains an additional component due to the ejection of ablated materials ("ablative-piston signal"); this propagates initially at supersonic speeds. The amplitude of the ablative-piston signal is found to be a monotonic function of the etch-depth per pulse. Hence, by accumulating the amplitude of the ablative acoustic pulse, the total etch depth can be monitored in real time.