Lithium tantalate resonant metasurfaces for highly efficient ultraviolet harmonic generation

Lithium tantalate (LiTaO3, LT), structurally similar to lithium niobate, possesses a broad spectrum of optoelectronic properties that hold significant promise for integrated photonics applications. Due to its larger bandgap and higher optical damage threshold, LT can be employed for efficient nonlinear optical frequency conversion in the ultraviolet (UV) wavelength range. Here, we report on fabrication of monolithic, high-quality LT metasurfaces, composed of periodic arrays of LT truncated square pyramids, created by partially etching of a LT thin film layer using a focused ion beam milling process. These pyramidal structures introduce discrete translational symmetry, which allows for the folding of guided-mode dispersion into the first Brillouin zone, leading to the formation of guided mode resonances with Q-factors that can be easily controlled by the height of LT pyramids. The measurements show a Q-factor up to approximately 640 at 786 nm. By leveraging the strong field localization of these high-Q resonances, we demonstrate enhanced second-harmonic generation at 393 nm with an absolute conversion efficiency of 0.042%, establishing a new benchmark for the UV harmonic generation in dielectric metasurfaces. Moreover, our fabrication technique can be readily adapted to a wide range of other material platforms, opening new avenues for the development of high-quality UV metasurfaces and advanced nonlinear photonic meta-devices.