Investigating focus elongation using a spatial light modulator for high-throughput ultrafast-laser-induced selective etching in fused silica

Ultrafast-laser-induced selective chemical etching is an enabling microfabrication technology compatible with optical materials such as fused silica. The technique offers unparalleled three-dimensional manufacturing freedom and feature resolution but can be limited by long laser inscription times and widely varying etching selectivity depending on the laser irradiation parameters used. In this paper, we aim to overcome these limitations by employing beam shaping via a spatial light modulator to generate a vortex laser focus with controllable depth-of-focus (DOF), from diffraction limited to several hundreds of microns. We present the results of a thorough parameter-space investigation of laser irradiation parameters, documenting the observed influence on etching selectivity and focus elongation in the polarization-insensitive writing regime, and show that etching selectivity greater than 800 is maintained irrespective of the DOF. To demonstrate high-throughput laser writing with an elongated DOF, geometric shapes are fabricated with a 12-fold reduction in writing time compared to writing with a phase-unmodulated Gaussian focus.