Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement.

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.

Ultra-smooth processing of lithium niobate for outstanding mid-infrared transmittance.

The fabrication of anti-reflection (AR) subwavelength structures (SWSs) of lithium niobate (LN) is a challenging but rewarding task in mid-infrared LN laser systems. However, there are still some issues with the high-quality processing and fabrication of bifacial AR SWSs. Herein, a novel, to the best of our knowledge, approach to the fabrication of SWSs was proposed, which includes femtosecond laser ablation followed by wet etching and thermal annealing. The fabricated structures exhibit high surface quality (Ra = 0.08 nm) and uniformity. According to the experimental and simulated results, the transmittance of the mid-infrared AR SWSs with a period of 1.8 µm could be improved from 78% to 87% in the 3.6-5 µm band. Furthermore, the double-sided construction enabled a transmittance of up to 90%. The results have great potential in the promotion of the development of mid-infrared laser systems and LN-based photonics.