Study on ablation threshold of fused silica by liquid-assisted femtosecond laser processing.

The femtosecond laser machining of fused silica in air and liquids is studied. The ablation threshold of fused silica is reduced from 2.22 to ${1.02}\;{{\rm J/cm}^2}$1.02J/cm. In order to explore the ablation mechanism fabricated in a liquid medium, the absorption characteristics of water and alcohol are studied. It is found that alcohol could absorb more laser energy than water. By analyzing the variation trend of laser-induced electron density based on an ionization model, we find that alcohol requires lower laser energy to reach the electron density standard and form plasma than water and fused silica. Besides, we observe that a laser will induce bubbles in liquids after the formation of plasma, and the bubbles in alcohol will cause stronger impact pressure to the surface of fused silica than those in water. Therefore, the mechanism of threshold reduction should be owed to the assistance of liquids with different characteristics.