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

ABSTRACT Confining light beyond the 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 a suspended thin‐film lithium niobate (TFLN) circular Bragg grating (CBG) cavity. The ultrasmall mode volume reaches less than . The CBG nanocavity exhibits a high normalized conversion efficiency of under the pump intensity of . 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.