Second-harmonic generation in monolithic lithium niobate metasurfaces

Second harmonic generation (SHG) in nonlinear metasurfaces has proven as a versatile tool for novel ultra-thin light sources with engineered spatial wavefronts. However, the availability of second-order nonlinear materials suitable for fabrication of such metasurfaces has so far been mainly limited to III-V semiconductors (e.g. GaAs, AlGaAs, GaP). Such systems have a limited transparency range and unusual off-diagonal second-order susceptibility tensors. These constraints have driven the search for new material platforms, including lithium niobate as one of the most promising nonlinear materials. Various approaches, such as plasmonic cavities [1], bottom-up techniques [2] and lithium niobate on insulator [3], have been recently explored, however with limited applicability. Here we propose a new design concept for second-order nonlinear metasurfaces on a monolithic substrate, which is not limited by the availability of thin crystalline films and can be applied to any non-centrosymmetric material. Our approach enables blue SHG from a monolithic lithium niobate metasurface with a conversion efficiency above 10∼⁵ using 1 GW/cm² pump intensity, opening new opportunities for practical designs of nonlinear metasurfaces.