Ultra-fast laser-induced processing of material: fundamentals and applications in micromachining

摘要

Fundamental questions arise regarding the possibility and nature of melting and the ensuing mechanism of ablation in femtosecond laser processing of materials. A comprehensive experimental study is presented to address these issues in depth and detail. The mechanisms of ultra-fast (femtosecond) laser-induced phase-transformations during the laser interactions with materials have been investigated by time-resolved pump-and-probe imaging in both vacuum and ambient environment. The temporal delay between the pump and probe pulses is set by a precision translation stage up to about 5400 ps and then extended to the nanosecond regime by an optical fiber assembly. Ejection of material in the form of nanoparticles is observed at several picoseconds after the main (pump) pulse. The ignition of surface-initiated plasma into the ambient air immediately following the pump pulse and the election of ablated material in the picosecond and nanosecond time scales have been probed by high-resolution, ultra-fast shadowgraphy. To further dissect the origin and evolution of the ablation process, a double pulse experiment has been implemented, whereby both the pump and probe pulses are split into two components each separated by variable temporal delays. A diffractive optical element is used to fabricate micro-channels in silicon wafers.