Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10-4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.
Keywords: Nonlinear optics; lithium niobate; membrane metasurface; resonance; second harmonic generation.