Topological photonic crystal fibers based on second-order corner modes

Photonic crystal fibers represent one of the most active research fields in modern fiber optics. The recent advancements in topological photonics have inspired new fiber concepts and designs. Here, we demonstrate a new, to the best of our knowledge, type of topological photonic crystal fiber based on second-order photonic corner modes from the Su-Schrieffer-Heeger model. Different from previous works where the in-plane properties at k_z=0 have been mainly studied, we find that in the fiber configuration of k_z>0, a topological bandgap exists only when the propagation constant k_z along the fiber axis is larger than a certain threshold and the emergent topological bandgap at large k_z hosts two sets of corner fiber modes. We further investigate the propagation diagrams, propose a convenient way to tune the frequencies of the corner fiber modes within the topological bandgap, and envisage multi-frequency and multi-channel transmission capabilities of this new type of fiber. Our work will not only have practical importance, but could also open a new area for fiber exploration where many existing higher-order topological photonic modes could bring exciting new opportunities for fiber designs and applications.