Interferometric probing of rapid vaporization at a solid–liquid interface induced by pulsed-laser irradiation

In situ monitoring of rapid vaporization process on a submicron length scale is essential for understanding the phenomena induced by pulsed-laser heating of solid surfaces in contact with liquids. A number of laser-processing applications including steam laser cleaning and liquid-assisted material ablation take advantage of enhanced acoustic excitation in the explosive vaporization process. In this paper, a novel technique based on Michelson interferometry is developed to study the liquid vaporization process induced by a KrF excimer-laser pulse (λ=248 nm, full-width-half-maximum (FWHM) = 24 ns) at relatively low fluences typically used for laser cleaning processes. The dynamics of rapid vaporization in a highly superheated liquid layer is measured in a semi-quantitative manner. In addition, optical-reflectance and forward-scattering measurements are performed to elucidate the vaporization dynamics. The results suggest that separate bubble nuclei begin to grow in the early stage of vaporization and tend to coalesce later. The maximum bubble size and growth rate are estimated to be of the order of 0.1 μm and 1 m/s, respectively. The bubbles decay at a significantly slow rate in comparison with the growth rate. The interference probe detects the long-term acoustic cavitation, so called memory effect, more effectively than the previously employed reflectance probe. This study reveals that the acoustic enhancement in the laser induced vaporization process is caused by bubble expansion in the initial growth stage, not by bubble collapse.