Femtosecond laser induced selective etching in fused silica: optimization of the inscription conditions with a high-repetition-rate laser source.

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.