Topological photonics is an emerging area that provides unprecedented opportunities for controlling the flow of light in photonic integrated circuits. With the introduction of non-trivial topological phases, a one-way street for light is feasible in photonic crystals (PhCs) and other platforms. Like a tightly regulated one-way traffic lane, light cannot be reflected back in these exotic structures.

However, such one-way transport of light at visible and near-infrared wavelengths may not robust against strong fabrication defects due to insufficient topological protection. Furthermore, poor mode confinement and limited bandwidth hinder the future development of high-density topological photonic integrated circuits.

To solve these problems, in a recent research paper published in ACS Photonics, LIU Tianji from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) collaborating with Satoshi Iwamoto from the University of Tokyo and Yasutomo Ota from Keio University numerically demonstrated the above 1,000-fold enlargement of topological bandgaps in epsilon-near-zero (ENZ) magneto-optical (MO) PhCs in comparison with previously reported results.

The proposed two-dimensional MO-PhC is composed of triangular MO prisms with a honeycomb lattice embedded in a silicon plate. With an applied magnetic field, non-trivial topological properties are imparted to the opening photonic bandgaps. In general, the topological gap size is extremely small at visible and near-infrared wavelengths, due to very weak responses in naturally occurring MO materials. On the contrary, with the help of artificial metamaterials, one can enhance MO responses by reducing diagonal permittivity elements of MO materials. As an extreme case, MO-PhCs with ENZ diagonal permittivity elements lead to a great enlargement of topological gap sizes.

A one-way street for light is built with the combination of two ENZ-MO-PhCs with the opposite magnetization. Unidirectional and backscattering immune transport of light is numerically obtained at the interface between two PhCs. The transport performance is unchanged even with large-size defects and sharp bends.

This work not only shows the opportunity to improve the performance of one-way light transport but also enriches the understanding of some fundamental phenomena?in topological photonics.