Second-order nonlinearity gives rise to many distinctive physical phenomena, e.g., second-harmonic generation (SHG), which play an important role in fundamental science and various applications. Lithium niobate, one of the most widely used nonlinear crystals, exhibits strong second-order nonlinear effects. However, its moderate refractive index and etching sidewall angle limit its capability in confining light into nanoscales, thereby restricting its application in nanophotonics.

The circular Bragg grating (CBG) cavity, known for its high light collection efficiency and vertical surface emission, finds broad applications in lasers, quantum emitters, and nonlinear devices. Additionally, the rotational symmetry of CBGs is anticipated to achieve polarization-independent performance.

Anapole resonance, without far-field radiation due to destructive interference of radiation patterns, is an ideal candidate for enhancing light-matter interactions at subwavelength scale. Here, we achieve anapole resonance enhanced SHG in CBG cavities on x-cut TFLN. The record-high normalized conversion efficiency is 1.21*10^-2 cm^2/GW under the pump intensity of 1.9 MW/cm^2, with an enhancement factor of 42000 compared to TFLN. Additionally, we also demonstrate s- and p-polarization-independent SHG in the elliptical Bragg grating (EBG) cavity, see also Fig. 1.

Fig. 1. (a) Schematic of CBG nanocavity on TFLN for enhanced SHG. (b) and (c) Electric and magnetic field distribution at the anapole resonant wavelength.

We leverage CBG nanocavities on TFLN to enhance SHG and have surpassed the current conversion efficiency in the nano-scale. A detailed comparison of our CBG nanocavity with its counterparts shown in Fig. 2.

Fig. 2. Comparison of SHG conversion efficiency of different structures.

Although the conversion efficiency of SHG can be enhanced by combining metal and LN, the conversion efficiency is still limited. The ohmic loss, zero bulk second-order nonlinearity of metals, and the low damage threshold of plasmonic structures result in a low nonlinear conversion efficiency. The moderate refractive index of LN, results in low Q factors of Mie resonance less than 100, which limits its SHG efficiency. Fano and anapole resonance structures can theoretically increase normalized conversion efficiency to 10^-5 cm^2/GW. Representative development is that the GMR and membrane metasurface structures increase the normalized conversion efficiency to 10^-5 cm^2/GW. Although the lithium niobate grating waveguide (LNGW) structure can theoretically achieve a conversion efficiency of 10^-3 cm^2/GW, it is difficult to carry out due to the stringent fabrication conditions. We achieve the highest normalized conversion efficiency on TFLN under the lowest pump intensity utilizing CBG nanocavities. Besides, CBG is extended to EBG to control the polarization of SHG, and s-/p-polarization independence is achieved without reducing the nonlinear conversion efficiency (order of 10^-2 cm^2/GW).

In summary, high-performance anapole resonance enhanced CBG nanocavities are designed on TFLN. We have also achieved SHG with s- and p- polarization independence in TFLN based on EBG nanocavities. The scheme can also be extended to other nonlinear optical platforms, such as transition metal dichalcogenides and III-V semiconductors. This work provides a new approach for studying nonlinear optics at the nanoscale void of phase matching.

This work is published in“Zengya Li, Zhuoran Hu, Xiaona Ye, Zhengyang Mao, Juan Feng, Hao Li, Shijie Liu, Bo Wang, Yuanlin Zheng, and Xianfeng Chen, Enhanced second-harmonic generation in thin-film lithium niobate circular Bragg nanocavity, Nano Letters 24, 11676-11682 (2024)”。