An ultrasmall-size topological dual-polarization beam splitter based on square lattice photonic crystal

Topological polarization beam splitters (TPBS) have extensive applications in laser systems and optical measurement systems. However, there are few photonic crystal (PC) structures based on this device in a two-dimensional square lattice. Herein, we propose an all-dielectric fishnet-type high-order topological photonic crystal (TPC) based on square lattice, which has the distinctive feature of having a common bandgap (CBG) for both transverse magnetic (TM) and transverse electric (TE) polarizations. It is uncommon in a square lattice. Based on the glide symmetry, two unit-cells are proposed, which have one-dimensional interface states and corner states in both polarization modes. The edge states in the two types of bent waveguides under the topological transport mode have extremely high transmission rates. Interestingly, when we broke the glide symmetry, we proposed a third unit cell, combined with two domain walls, and found that only one of the TM and TE polarized waves could pass through each of them. Thus, a TPBS is constructed. Simulation results prove its excellent beam splitting capability. Moreover, this TPBS system exhibits significant robustness in the face of minor disturbances and defects. Compared with the previous TPBS devices, considering the material loss and reflection conditions in practical applications, our system based on a square lattice design has a smaller footprint, higher-order corner state properties, and the topological edge states have lower transmission losses. This enables the system to achieve low-loss anti-scattering light transmission in photonic chips and at the micro-nano scale.