MICROSCALE PHENOMENA DURING LASER-LIQUID INTERACTIONS

The interaction of high-intensity, pulsed laser radiation with liquids is central to many contemporary technologies, including laser desorption of liquid films, laser cleaning and particle removal, laser surgery, and laser water removal from microelectronics and MEMS devices. The liquid phase is also present during laser melting and vaporization processes. Short-pulse, high-intensity lasers are able to generate intensities that far exceed the threshold intensities for nonlinear light-matter interactions in liquids. Classical models of absorption and heating fail to capture high-intensity, nonlinear phenomena in liquids. Furthermore, at very high intensities, the actual liquid behavior can deviate significantly from classical model predictions. This work develops a thermal model for high-power, short-pulse laser heating of liquids that accurately predicts the temperature distribution in the liquid when intensity-dependent effects occur, and provides simple, quantitative criteria to determine when such effects are important. Model results are used to characterize laser-liquid interactions as a function of laser pulse duration, intensity, wavelength, repetition rate, and liquid properties. For liquids that normally absorb at the laser wavelength, high incident laser intensities can promote a large number of molecules to excited states, which have different absorption characteristics, resulting in significant changes in absorption and heating in the liquid. This effect, called saturable absorption, is exhibited by many liquids, including water. At high intensities, the temperature rise can be up to two orders of magnitude less than that predicted by classical models. This information is important for temperature-sensitive aspects of laser-liquid applications, including maximum temperature rise, phase change, and thermophysical property changes. A phenomenon exhibited at high intensities in transparent liquids is multiphoton absorption and ionization, in which enough photons are absorbed lo ionize the liquid molecule. In particular, the electron removed during ionization absorbs extremely strongly and has a very long lifetime. The enhanced absorption due lo ionized electrons provides a new mechanism for depositing large amounts of thermal energy into otherwise transparent liquids. Results arc presented for water irradiated by a Nd:YAG laser. Such strong absorption is important for applications where laser heating of the liquid is to be maximized, for example, laser desorption of thin liquid films.