High-quality-factor lithium niobate photonic crystal cavity can trap photon within small space effectively and shows potential in high-quality modulator, sensor and nonlinear frequency conversion. Large amounts of lithium niobate photonic crystal cavities are usually constructed by electron beam lithography together with reactive ion etching. Lithium niobate is hard to etch, as a result, the fabricated air hole usually has 70 degrees slant angle, which can twist the light into substrate direction and affect the localization of photon. Consequently, lithium niobate photonic crystal cavity usually shows low quality factor compared with silicon photonic crystal cavity. Researchers proposed many methods like heterostructure and inverse design to enhance the quality factor of lithium niobate photonic crystal, which require numerous trial and error simulations.

To solve the problem, we cascaded several bichromatic lithium niobate photonic crystal cavities. The constructed cavity has ~1.4×105 quality factor, which is 6 times larger than the one in isolated cavity (Q~2×104). The bichromatic photonic crystal cavity is constructed by a traditional photonic crystal with a line defect with another extra photonic crystal introduced. The added photonic crystal and the original photonic crystal have different lattice constants. By analyzing the space Fourier-transform-spectra of the fields in isolated and cascaded photonic crystals, we find cascading can filter out the energy in spectra periodically. For cascaded photonic crystal, the implement lower the relative energy within the light cone region in spectra. We fabricated the proposed lithium niobate photonic crystal to verify the feasibility of our design. Our work can provide guidance to the design of high-quality factor lithium niobate photonic crystal cavity.

图1 (a) 所提出级联双色铌酸锂光子晶体腔结构图。(b)所提出光子晶体腔透射谱及模场分布。(c)所提出光子晶体腔的空间傅里叶变换谱。(d)电子束曝光结合反应离子刻蚀在铌酸锂薄膜上制备的71°孔洞。(e)制备的级联双色铌酸锂光子晶体腔。

This research is published in “Rui Ge, Xiongshuo Yan, Zhaokang Liang, Hao Li, Jiangwei Wu, Xiangmin Liu, Yuping Chen and Xianfeng Chen. Large quality factor enhancement based on cascaded uniform lithium niobate bichromatic photonic crystal cavities, Optics Letters, 48(1), 113-116 (2023)”.

Link: https://doi.org/10.1364/OL.477895