Ultrahigh-speed micromachining of sapphire by enhancing laser absorption.

Abstract

Fine micromachining of sapphire using ultrafast lasers via nonlinear absorptions has been extensively studied. However, severe damage, along with the low material removal rate, limits the precision and machining speed. Here we use transient and selective laser processing for sapphire by coaxially delivering a single femtosecond laser pulse and a single microsecond laser pulse that are temporally synchronized. It shows that the micromachining speed was improved 10⁴ times, and cracks in the base material are mitigated, compared to femtosecond laser percussion processing with a repetition rate of 1 kHz. The ultrafast electron excitation dynamics induced by the femtosecond laser are captured using pump-probe imaging, revealing a rapid relaxation process with a decay time constant on the order of picoseconds. The machining process is further visualized using a high-speed camera and compared with numerical simulations to elucidate the underlying mechanism. We find that the increased absorption coefficient of sapphire for the microsecond laser pulse, owing to the bandgap narrowing and thermal ionizations, along with high-temperature regions expanding from the sample surface into deeper areas along the laser propagation path, drives the machining process. These results advance high-efficiency and precision micromachining for sapphire and other hard and brittle transparent materials.