Highly confined out-of-plane corner states in air-hole type topological photonic crystal fiber

Abstract

Out-of-plane corner states are found in air-hole type square lattice topological photonics crystal fiber (PCF) based on highly nonlinear glass. While, for the unit cell of photonic crystal (PC) without lattice shift (UC1), the parities of Bloch modes under the first photonic bandgap (PBG) are reversed between and X points in the Brillouin zone, the parities are preserved for the unit cell with lattice shift (UC2). This is opposite compared with the case of pillar type topological PCFs. Based on these results, out-of-plane optical states of the fiber with three UC1 and one UC2 regions are investigated, and two highly confined corner states are found at the corner of UC1 and UC2 interface. The corner states are not found by just flipping unit cell, namely, for three UC2 and one UC1 PC corner (for pillar type PCF, the corner states were found for this placement). Wavelength dependence of effective area and the confinement loss of the corner states are investigated in detail, and by increasing the number of layers, the confinement loss can be dramatically reduced. These new fibers may be applied to nonlinear optical applications, such as wavelength conversion and single photon generation.journal articl