Femtosecond Laser-Induced Periodic Surface Nanostructuring of BaGa₄Se₇ Crystal for Near-Infrared Anti-Reflection Enhancement

Efficient near-infrared (NIR) optical devices based on non-linear crystals are demanded for diverse applications ranging from bio-imaging and diagnostic tools to optical parametric conversion. However, significant reflection energy losses, caused by high refractive index of the crystal material, limit their performance. Here, based on example of promising BaGa₄Se₇ nonlinear crystals we demonstrated simple, high-performing and single-step approach to fabricate anti-reflective structures onto their surfaces through direct femtosecond-laser multi-pulse processing. We found that under multi-pulse exposure laser-induced periodic surface structures (LIPSS) with nanotrenches period around 380 ± 10 nm and orientation perpendicular to the irradiating laser beam polarization can be created. The minimal structural damage of the laser-patterned crystals was justified by combining transmission electron microscopy with Raman/X-ray diffraction spectroscopies. Fourier-transform infrared spectroscopy was revealed 1.12-fold transmission increase within the 1.3–2 µm spectral range for LIPSS-textured BaGa₄Se₇ crystal as compared to the untreated one. The proposed method to enhance the efficiency of BaGa₄Se₇ non-linear optical crystal in the practically relevant NIR spectral range holds significant potential for non-linear optical applications.