Coaxial nanopatterned lithium niobate embedded in Ag enables a large electromagnetic confinement in small-volume cavities. The nonlinear material filling these cavities is at the origin of high second-order nonlinear conversion, where no phase matching is needed. A doubly resonant optical spectrum of the linear response is required in order to boost this phenomenon, providing a promising and stable second-harmonic (SH) device tailored for any desired wavelength. The structure is fabricated by using electron-beam lithography, dry etching, and chemical mechanical polishing. We report a second-harmonic generation (SHG) enhancement factor of 26 compared to unpatterned -cut lithium niobate wafer at . The enhancement, which comes exclusively from the nanostructured lithium niobate, is experimentally and theoretically demonstrated. A comparison with three types of metallic subwavelength apertures is shown. The embedded structure shows the strongest SH signal. Analysis of the strong dependence of the incident polarization on the SHG intensity with a homemade 3D–NL–FDTD algorithm shows good agreement with experimental data.
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