Design of broadband all-dielectric valley photonic crystals at telecommunication wavelength

Abstract

Topological photonic crystals (TPC) can effectively suppress the backscattering from defects and achieve unidirectional transmission with high forward transmittance, thus becoming a promising candidate in integrated photonic chip applications. Among different TPCs, the valley photonic crystals (VPCs) based on quantum valley Hall effect (QVHE) originating from valley-dependent spin-split bulk bands, can achieve spin-dependent unidirectional transmission at telecommunication wavelength with conventional dielectric material, such as silicon, which can be readily fabricated by current CMOS nanofabrication techniques. Therefore, VPCs recently attract broad attention. Generally, a large working bandwidth is preferred in optical communications and information processing, which should be as large as possible. Currently, the maximum working bandwidth of VPCs at telecommunication wavelength is limited to about 130 nm. Based on theoretical analysis and numerical simulation, a triangular lattice VPC composed of all-dielectric silicon-based triangular air holes is proposed, and the working bandwidth at 1550 nm is further extended to 229 nm. By combining two mirror-inversed VPCs, we observe the valley–spin locking behavior results in selective net spin flow inside bulk VPCs to achieve unidirectional transmission with a high forward transmittance up to 0.97. Moreover, the intensity distributions of the incident light show neglectable scattering loss at the sharp edge of the zigzag and Ω shape waveguides, confirming the achievement of scattering immune propagation. The designed VPCs not only offer a possibility to expand the working bandwidth, but also can be applied to device applications in integrated photonics and information processing using spin-dependent transportation.