Micromachining by laser ablation of liquid: Superheated liquid and phase explosion

Transparent materials such as quartz, calcium fluoride and fluoropolymer, are difficult to fabricate by conventional laser processing. We fabricated them successfully with a micron-size by laser ablation of liquid layer attached on the backside of the target. The threshold fluence for etching depends on absorption coefficient of liquid, and thermal properties of target materials. In the further study, we observed the dependency of the etch rate on etch size. The results suggest that the thermal energy diffuses more efficiently when the irradiation area is too small. However, when increase the etch depth to high aspect ratio, in the strong laser fluence, the etch rate becomes faster than that with large irradiation area and low aspect ratio. We discussed that the mechanism is due to the combination of two processes in the interface of a target material and a liquid: one is a heating process (phase explosion, explosive boiling) by the superheated liquid and the other is an attacking process by high temperature and pressure. We suggested that the temperature of the superheated liquid can exceed the thermodynamic critical temperature Tc upon irradiation at a high laser fluence, and the liquid corresponded to "transient supercritical fluid". The generation of super-heated liquid is based on a enormous heat release from organic molecule by a cyclic multiphoton absorption mechanism.