Abstract
Miniaturizing nonlinear optical components is essential for integrating advanced light manipulation into compact photonic devices, enabling scalable and cost-effective applications. While monocrystalline lithium niobate thin films advance nonlinear nanophotonics, their high inertness limits the design of top-down fabricated nanostructures. A versatile bottom-up fabrication method based on nanoimprint lithography is presented for achieving polycrystalline lithium niobate nanostructures and demonstrate its significant potential for nonlinear metasurfaces. The fabrication enables nearly vertical features and aspect ratios of up to 6 combined, which we combine with a novel solution-derived material with high effective second-order nonlinearity d(eff) of 5 pm V(-1). On this platform, second-harmonic focusing is demonstrated over a broad spectral range from near-ultraviolet to near-infrared, increasing the nonlinear signal intensity by up to 34 times. This method enables the first lithium niobate metalens and expands the field of nonlinear metasurfaces by providing a low-cost, highly scalable fabrication method for engineered nonlinear nanostructures.