UltravioletMetalens Based on Nonlinear WavefrontManipulation of Lithium Niobate Metasurfaces

Nonlinear metalens has revolutionized the generation and focusing of nonlinear harmonic waves, significantly enhancing the performance of nonlinear frequency converters and expanding the applications of metalens from linear optics to nonlinear optics. However, current nonlinear metalens are typically constructed using low-nonlinear-susceptibility materials, impeding further advancements in the field. Here, we have developed a nonlinear metalens utilizing crystallized lithium niobate (LN), a material with a large second-order susceptibility and a broad transparent window. The meta-atoms of the LN metalens consist of elliptical nanoholes, fabricated by using a self-developed multiatmosphere cooperative etching technique. With a diameter of 200 μm, our fabricated metalens can focus the second harmonic wave at 390 nm to a 0.7-μm-wide spot with a focus length of 100 μm, resulting in a numerical aperture of 0.7. The peak intensity of the focus plane is enhanced by 40 times. Our LN metalens has potential applications in versatile nonlinear meta-optic devices for highly efficient frequency converting, high-resolution imaging, and bright entangled photopairs.