Observation of Anapole Resonances in Lithium Niobate Metasurfaces with Significantly Enhanced Second Harmonic Generation

Benefiting from their large second-order nonlinear coefficients and high integration capabilities, crystalline lithium niobate (LN) films have shown great application prospects in the field of nonlinear metasurfaces. It is necessary to endow LN metasurfaces with optical resonances to further boost nonlinear optical responses. Among these optical resonances, anapole resonances carried by LN metasurfaces have been predicted to efficiently enhance second harmonic generations (SHG) but have never been experimentally realized. Anapole resonance requires ideal nanostructures to have steep sidewalls and a high filling ratio, but the intrinsic hardness and inert chemical properties of LN materials pose great challenges for the fabrication of LN nanostructures. Here, a multi-gas component dry-etching technique is proposed to prepare various LN nanostructures, achieving typical LN nanopillar arrays with a sidewall angle of ≈85° at a depth of 300 nm and a filling ratio of 52%. Importantly, nanostructured LN metasurfaces are designed and fabricated to experimentally realize anapole resonances at a fundamental wavelength of ≈800 nm, demonstrating a ≈30-fold enhancement in second harmonic generation compared with bare LN films. The work provides promising strategies for the versatile fabrication of LN nanostructures and their applications in nonlinear meta-optics.