Light confinement at the deep-subwavelength, and even atomic scale, has long represented a fundamental challenge and a focal point of ongoing research in nanophotonics. The ability to spatially localize photons with extreme precision has opened new avenues for the realization of nanoscale lasers, single-molecule sensing, superresolution imaging, as well as high-efficiency nonlinear and quantum optical devices. Achieving such confinement typically requires carefully engineered nanostructures by leveraging optical resonances that can overcome the optical diffraction limit while minimizing optical losses. Plasmonic resonances, which achieve strong field confinement by coupling light with the collective oscillation of free electrons in metals, can break the optical diffraction limit and enhance light-matter interactions at deep-subwavelength scales. Although plasmonic structures possess small mode volumes, their high ohmic loss, low damage thresholds, and lack of bulk second-order nonlinearity often result in limited nonlinear conversion efficiency. 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 0.001(λ/n)^3. The CBG nanocavity exhibits a high normalized conversion efficiency of  0.85×10^-2 cm^-2·GW^-1 under the pump intensity of 1 MW·cm^-2. This approach paves the way for nonlinear nanodevices for robust sub-diffraction light–matter interaction in an ultra-compact and lossless dielectric platform.

Fig. 1. Schematic and mode distribution comparation of CBG structure and bowtie embedded in CBG. (a) Schematic of suspended CBG. (b-d) Electric and magnetic field distribution of CBG. (e) Schematic of bowtie structure embedded in CBG. (f) Electric field compression in bowtie. (g) Measured optical quality factors.

The FH pump corresponding to the structural resonance under different periods and duty cycles is extracted to evaluate the enhancement of the SHG signal. A maximum enhancement factor of up to 3,720 is achieved for the bowtie-in-CBG structure compared to bare TFLN. In the design, the Bowtie structure is required to be minimized in size. For structures with small lattice periods and low duty cycles, the introduction of bowtie-shaped elements can significantly alter the resonant conditions of CBG. Consequently, the introduction of Bowtie-in CBG does not yield a noticeable enhancement under certain period and duty-cycle conditions. However, a pronounced enhancement effect is observed at large periods and high duty cycles.

Fig. 2. SHG enhancement in the TFLN nanocavity sample. (a) Spectral and temporal profiles of the pump. (b) SHG enhancement with different periods. (c) SHG enhancement with different duty cycles at a fixed grating period of 760 nm. (d) Experimentally observed visible SHG signal.

This work is published in “Sub-Diffraction Optical Confinement for Enhanced Second-Harmonic Generation in Suspended Thin-Film Lithium Niobate Nano-Cavity. Laser & Photonics Reviews (2026): e71263.”

Link: https://onlinelibrary.wiley.com/doi/10.1002/lpor.71263