One-Step Fabrication of Non-Ablative Microstructures for Efficient Second Harmonic Generation on LiNbO₃ Thin Film

Lithium niobate (LiNbO₃) is a key material for nonlinear optics applications, such as second harmonic generation (SHG), due to its strong nonlinear coefficient and broad transparency window. However, its high damage threshold makes it difficult to fabricate high-quality microstructures, often resulting in low quality factors and poor SHG efficiency. Femtosecond lasers are well suited for processing transparent materials, but conventional ablation typically introduces surface cracks and scattering centers that degrade optical performance. In this study, we develop a one-step femtosecond laser modification method to create periodic metasurfaces on LiNbO₃ thin films without ablation. Compared to ablated structures, the laser-modified regions exhibit minimal surface damage and avoid microcracking. The designed periodic structures facilitate efficient second-harmonic generation through complementary mechanisms: the laser-modified regions introduce a longitudinal focusing effect that locally enhances the optical field, whereas the periodic index modulation enables grating coupling to guided-mode resonances under oblique incidence. Experimental results demonstrate a 4.75-fold enhancement in SHG efficiency under 1030 nm femtosecond laser irradiation at oblique incidence. The mechanisms are analyzed using finite-difference time-domain simulation and Raman spectroscopy characterization. This work provides a simple and damage-free strategy for manufacturing high-performance nonlinear photonic devices, facilitating applications in integrated photonic circuits.