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

Topological photonics provides exciting and emerging opportunities for the manipulation of light. As the photonic analogue of quantum Hall edge states, chiral edge modes, arising at the interface between two photonic topological structures with different Chern numbers, hold great promise for robust transport of light against disorders and defects. However, for magneto-optical material-based topological photonic crystals, the transport performance of chiral edge modes is strongly dependent on the topological gap sizes, which are usually very narrow at optical frequencies due to the lack of magneto-optical materials with strong nonreciprocal responses. Here, we numerically demonstrated that the introduction of an epsilon-near-zero effect to magneto-optical photonic crystals could remarkably enlarge topological gap sizes due to the boosted magneto-optical response. Eigenmode calculation results show that the boosted magneto-optical response correlates to the enhanced nonreciprocal power flows in magnetized photonic crystals with an epsilon-near-zero diagonal permittivity. The enlarged topological band gap leads to the broadband and well-confined chiral edge modes propagating along the magnetized boundary between two oppositely magnetized photonic crystals. More importantly, such mode propagation shows strong robustness against sharp bends and large defects. In principle, our proposal for the enlargement of topological photonic band gaps could also be valid in photonic crystal slabs or even three-dimensional photonic crystals. Our results not only suggest the possibility to improve the transport performance of one-way modes in magneto-optical photonic crystals but also enrich the physical understanding of the epsilon-near-zero effect-based topological photonics.