Topological Band Gaps Enlarged in Epsilon-Near-Zero Magneto-Optical Photonic Crystals

Topological photonics provides exciting and emerg-ing opportunities for the manipulation of light. As the photonicanalogue of quantum Hall edge states, chiral edge modes, arising atthe interface between two photonic topological structures withdifferent Chern numbers, hold great promise for robust transportof light against disorders and defects. However, for magneto-opticalmaterial-based topological photonic crystals, the transport perform-ance of chiral edge modes is strongly dependent on the topologicalgap sizes, which are usually very narrow at optical frequencies dueto the lack of magneto-optical materials with strong nonreciprocalresponses. Here, we numerically demonstrated that the introduc-tion of an epsilon-near-zero effect to magneto-optical photoniccrystals could remarkably enlarge topological gap sizes due to the boosted magneto-optical response. Eigenmode calculation resultsshow that the boosted magneto-optical response correlates to the enhanced nonreciprocal powerflows in magnetized photoniccrystals with an epsilon-near-zero diagonal permittivity. The enlarged topological band gap leads to the broadband and well-confinedchiral edge modes propagating along the magnetized boundary between two oppositely magnetized photonic crystals. Moreimportantly, such mode propagation shows strong robustness against sharp bends and large defects. In principle, our proposal for theenlargement of topological photonic band gaps could also be valid in photonic crystal slabs or even three-dimensional photoniccrystals. Our results not only suggest the possibility to improve the transport performance of one-way modes in magneto-opticalphotonic crystals but also enrich the physical understanding of the epsilon-near-zero effect-based topological photonics