Out-of-plane corner states in topological photonic crystal fiber with all-circular air-holes

Out-of-plane corner states are found in a topological photonics crystal fiber (TPCF) composed of all-circular air-holes based on a square lattice with highly nonlinear glass. The condition for topological corner states exists between the unit cell of PC having a circular air-hole at its center (UC1), and the unit cell with a half-period shift (UC2). The topological corner state is found at the corner of TPCF, composed of UC1 and UC2. Since UC2 is composed of four quarter circles, the TPCF includes quarter and half circles. Since it is difficult to retain the shape under the fabrication, the corner mode of TPCF composed of all-circular air-holes, in which quarter and half circles are replaced with a perfect circle and an ellipse, is also investigated. Highly confined corner states are found for all-circular TPCF, even though the interface between UC1 and UC2 is no longer a precise photonic crystal boundary. By increasing the diameter of the air-hole, < 0.1 μm 2 confinement is possible, leading to the giant enhancement of the nonlinear optical effect. Furthermore, twisted guided modes of all-circular TPCF are investigated for the first time, and it is demonstrated that the twisted corner modes are robust to the twisting of the fiber. These findings are important for the actual application of TPCFs.