The topological valley photonic crystal has recently attracted extensive attention owing to its superior properties like possibility to produce edge state below the light line and showing ultra-low loss when passing 120° sharp bends. Designing broadband and lossless valley photonic crystal waveguide is an important goal for researchers. Methods such as changing the shape of holes and inverse design have been used to achieve this goal. Now that broadband lossless valley waveguide is merely achieved based on Si valley photonic crystal with high refractive index. Thus achieving broadband valley photonic crystal waveguide based on materials with relatively low refractive is extremely necessary. For instance, valley photonic crystal based on BaTiO3 has already been used to construct adjustable optical switch and valley photonic crystal based on SiN has already used to construct waveguide coupler for single WSe2 monolayer.
The research group lead by Prof. Yuping Chen and Xianfeng Chen from Shanghai Jiaotong University proposed the band property of valley photonic crystal based on lithium niobite (R. Ge, et al, Broadband and lossless lithium niobate valley photonic crystal waveguide,Chinese Optics Letters, 19(6)). A chiral and lossless valley photonic crystal waveguide has been achieved which is robust to Z bend waveguide. And the corresponding bandwidth is 0.0201(a/λ), which is not easy to realize in low-refractive index material. In this case, the edge state is possible to locate below the light line only when the interface is arranged in bearded type, and top and bottom components show negative and positive valley Chern numbers. In this condition, edge state band actually includes two different sub-bands. While the edge state band in lower frequency cannot pass through 120° sharp bends, which is topologically trivial. However, the two sub-bands are locked to the Bloch modes with the same chirality.The results can be extended to other materials that have low refractive index such as LiTaO3, BaTiO3, SiN, etc.
Schematic of the "bearded" interface lithium niobate valley photonic crystal, and the corresponding group index diagram, projected band diagram, |Hz| field distribution and field distribution for straight and bend waveguide.
