In-situ enhanced laser absorption in aqueous transition metal salt solution enables high-quality backside wet etching of optical glass by near-infrared lasers

A laser-induced backside wet etching technique using near-infrared laser sources (N-LIBWE) has been proposed for high-quality micromachining of optical glass. Compared with conventional LIBWE using organic dye solutions with high absorption coefficients (∼104–105 cm−1) as the liquid absorbent, the transition metal salt solution used in N-LIBWE shows the absorption coefficient of only 10-1-101 cm−1. Moreover, as we proved in this study, the etching rate of N-LIBWE is independent of the absorptivity of the liquid absorbent. How the etching occurs during N-LIBWE is still controversial. Herein, interface images with a temporal resolution of 4 ns after the start of laser irradiation are captured by a stroboscopic shadowgraphy system. Afterward, the density, pressure, and temperature distribution in the solution are calculated by a physics-based two-dimensional hydrodynamic model to explain the observed interface images. We prove that the localized laser absorption is enhanced with laser irradiation because of the in-situ formation of solid substances. It is the properties of the formed solid products instead of the normal absorption coefficient of the solution that determines the final interface status. We further prove that the formed solid substances in aqueous CuSO4 solutions are primarily amorphous nanoclusters, which benefit the enhancement of laser absorption and tend to attach to the transparent substrates, resulting in the incubation effect in N-LIBWE. Our results provide systematic mechanisms explaining the N-LIBWE process, which also benefit the deep understanding of other liquid-assisted laser materials processing technologies.