Efficient and Tunable Narrowband Second‐Harmonic Generation by a Large‐Area Etchless Lithium Niobate Metasurface

Boosting the efficiency of nonlinear optical processes at the nanoscale is essential for realizing scalable platforms for frequency conversion, nonlinear sensing, and photon‐pair generation. However, the drastic reduction of material volume at subwavelength dimensions fundamentally limits conversion efficiency. Nonlocal optical resonances, such as quasi‐bound states in the continuum (q‐BICs), can overcome such constraints thanks to enhanced light–matter interaction associated with their high quality ( Q ) factors. Together with recent advances in thin‐film lithium niobate (TFLN), they establish a powerful platform for compact and efficient nonlinear photonic devices. Here, we demonstrate a large‐area metasurface‐enabled TFLN platform implemented with a slanted TiO 2 nanograting fabricated via scalable nanoimprint lithography. The structure supports q‐BICs that enable efficient coupling to a narrowband pulsed pump for spectrally selective second‐harmonic generation (SHG). We achieve a normalized SHG efficiency of 0.15% cm 2 /GW, among the highest reported for lithium niobate metasurfaces, at a low pump peak intensity of 3.64 kW/cm 2 , enabling operation even under continuous‐wave excitation. Additionally, wavelength tuning from 870 to 920 nm with stable output power and polarization control is achieved via off‐normal illumination. This scalable platform opens opportunities for sensing, terahertz generation and detection, and ultrafast electro‐optic modulation.