Configurable topological beam splitting via antichiral gyromagnetic photonic crystal

Topological insulators, whose bulky states are prohibited while surface/edge states are conductive and topologically protected. Recent advances in topologically protected edge states have drawn growing attention in the optics and photonics community. In 2008, Raghu and Haldane first theoretically predicted that a topologically protected chiral one-way edge state can be created by analogy to the integer quantum Hall effect in a two-dimensional electron gas system, where the one-way edge states propagate along the opposite directions at two parallel edges of a gyromagnetic photonic crystal [Phys. Rev. Lett. 100, 013904 (2008)].

In 2020, the research group of Prof. Zhi-Yuan Li from South China University of Technology theoretically proposed another intriguing case where the one-way edge states at two opposite parallel zigzag edges can propagate in the same direction, and they are called antichiral one-way edge states [Phys. Rev. B 101, 214102 (2020)]. To date, antichiral one-way edge states have been studied in various fermionic and bosonic systems; however, many of studies only focused on the demonstration of antichiral one-way transport property, and few of them touch on the unique properties of antichiral topological systems and novel applications.

A new Opto-Electronic Science study reports the construction and observation of topological beam splitting with an easily adjustable right-to-left ratio in an antichiral gyromagnetic photonic crystal. The splitter is compact and configurable, has high transmission efficiency, allows for multi-channel utilization, is crosstalk-proof, and is robust against defects and obstacles. This performance is attributed to the peculiar property that antichiral one-way edge states exist only at the zigzag edge but not at the armchair edge of antichiral gyromagnetic photonic crystal. When they combine two rectangular antichiral gyromagnetic photonic crystals holding left- and right-propagating antichiral one-way edge states, respectively, bidirectionally radiating one-way edge states at two parallel zigzag edges can be achieved. Finally, the researchers designed a topological beam splitting with the configurable splitting ratio that can easily be adjusted by simply changing the source excitation condition. These observations can enrich the understanding of fundamental physics and expand topological photonic applications.