Sub-Diffraction Optical Confinement for Enhanced Second-Harmonic Generation in Suspended Thin-Film Lithium Niobate Nano-Cavity

Confining light beyond diffraction limit is critical in a plethora of important applications, including nano-lasing, sensing, imaging, and particularly nonlinear optics, as it is essential for enhancing conversion efficiency. Plasmonic resonances can provide superior field confinement, but inherently suffer from ohmic losses and thermal damage. Implementing deep-subwavelength optical confinement in all-dielectric materials for enhanced light-matter interactions devoid of these drawbacks is appealing yet challenging. Here, we demonstrate significant enhancement of second-harmonic generation (SHG) in a bowtie nanostructure embedded within suspended thin-film lithium niobate (TFLN) circular Bragg grating (CBG) cavity. The ultrasmall mode volume reaches less than $0.001 (\lambda/n)^3$. The CBG nanocavity exhibits a high normalized conversion efficiency of $0.85 \times 10^{-2}\ \text{cm}^2/\text{GW}$ under the pump intensity of $1\ \text{MW}/\text{cm}^2$. An SHG enhancement factor of approximately 3,720 compared to bare TFLN is realized. This approach paves the way for nonlinear nanodevices for robust sub-diffraction light–matter interaction in an ultra-compact and lossless dielectric platform.