Femtosecond‐Laser‐Induced Selective Etching in a Mid‐Infrared Glass

Femtosecond‐laser‐induced selective etching (FLISE) has been extensively studied and optimized in fused silica, enabling precise 3D microstructuring in the visible and near‐infrared spectral ranges. Extending this approach to mid‐infrared (MIR) transparent materials remains an important challenge for enabling new photonic and optofluidic functionalities. Here, we report the first demonstration of selective etching in a gallate glass composition transparent up to ~6 µm, following ultrafast laser modification. Among the investigated acids and bases, HCl emerges as the most effective etchant. Using a repetition rate of 100 kHz, pulse energies of 0.7–0.85 µJ, and perpendicular polarization, etching rates exceeding 4000 µm h ⁻¹ with aspect ratios up to ~15 are achieved in 1 mol L ⁻¹ HCl. Thermal modeling and annealing experiments reveal the existence of a narrow processing window below ~300–350 °C, suggesting the involvement of oxygen‐related defects in the etching mechanism. These results provide a first framework for understanding and optimizing FLISE in a gallate glass composition and demonstrate that this class of materials is promising for 3D microstructuring in the mid‐infrared spectral range.