Highly Efficient Ultraviolet Harmonic Generation Based on Coupled Guided Mode Resonances in Lithium Niobate Metasurfaces

As one of the most versatile and attractive materials for photonics, lithium niobate (LiNbO3, LN) exhibits exceptional electro‐optic, nonlinear‐optic, and acousto‐optic properties, along with a bandgap of 3.93 eV (315 nm), designating it as an ideal candidate for nonlinear optics within the near‐ultraviolet (near‐UV) spectral region (320–400 nm). However, efficient UV emission from LN nonlinear metasurfaces remains rare, primarily due to the challenges associated with achieving high‐quality nano‐structuring of LN meta‐atom sizes comparable to UV wavelengths. This study investigates the design and performance of high‐quality LN metasurfaces optimized for enhanced nonlinear optical interactions, particularly second harmonic generation (SHG) in the near‐UV range. The designed metasurface consists of arrayed nano‐disks created through the partial etching of a LNOI thin film layer. These nano‐structures introduce discrete translational symmetry, enabling the folding of guided‐mode dispersion into the first Brillouin zone and facilitating the formation of high‐quality‐factor guided mode resonances (GMRs). Importantly, significant field localization and strongly coupled resonances are demonstrated by finely tuning the coupling strength of two spectrally close GMRs, resulting in enhanced SHG at 394 nm with a record‐high absolute conversion efficiency of 1.36 × 10−3. These findings underscore the potential of GMR‐based LN metasurfaces for advancing nonlinear optical applications and highlight their effectiveness in efficiently harnessing light‐matter interactions at the nanoscale.