Helical edge states of photonic crystals show promise for optical integrated circuits

Topologically protected edge states of photonic crystals have been studied extensively for their electromagnetic properties. However, they also possess intriguing optical properties, such as coupling.

To further explore these phenomena, a team of researchers constructed a two-dimensional “sandwich” photonic crystal based on a trivial-nontrivial-trivial topology. Gao et al. report that the height of the layer with nontrivial topology is a parameter that can be used to generate the two edge states and control the coupling between them. This results in unidirectional light propagation in the nontrivial topological region. Inside this region, robust light confinement and protection is observed.

This finding is significant because topological edge states are key components in integrated optics due to the quantum spin Hall effect. A major advance would be realized if structures for coupling two helical edge states could be achieved because, being topologically protected, the states remain stable and are immune to imperfections. As such, they may be suitable for applications where structures must be flexible.

In the reported configuration, two helical edge states were excited simultaneously by one source, and had similar properties for coupling thanks to the symmetry of the band structure. According to the team, these states provide robust light confinement and protection properties. In particular, the unidirectional propagation attribute is an effective method of guiding waves in optical circuits, all-optical circuitry and devices for optical communication.

Source: “Unidirectional propagation of coupled edge states in the sandwich topological photonic crystals,” by Yong-Feng Gao, Zhen Jiang, Lu-Lu Zhang, Liu He, and Jian Zhao, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5047647 .