Abstract

The interaction between high-intensity laser pulses and surfaces generates acoustic pulses in ambient air, which can be monitored by deflection of a He–Ne laser beam. With this photoacoustic deflection technique, surface damage thresholds can be reliably determined. Moreover, the photoacoustic signals should bear information about the physical processes dominating the laser–surface interaction. With this goal in mind we applied the photoacoustic technique to various surfaces, ranging from metals to polished transparent materials. For 2–3-ns laser pulses and wavelengths between 510 and 560 nm, we find clear evidence for multiphoton absorption in the case of metals and rough surfaces of transparent materials. The accumulated energy of the absorbed photons corresponds to the work function or the band gap of the respective material, indicating that the primary excitation mechanism is hole creation by multiphoton ionization, followed by secondary processes which, of course, differ greatly for different materials. In contrast, polished surfaces of transparent materials show higher damage thresholds. For the most inert surfaces, e.g., CaF₂ and MgF₂, the respective integrated photoacoustic signals vary more dramatically with laser intensity, suggesting that in these cases dielectric breakdown is the dominant process.

© 1988 Optical Society of America