Recently, researchers from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, cooperating with researchers led by Prof. ZHANG Shuang from the University of Birmingham in the UK, have theoretically studied the surface states between two topological non-trivial photonic systems. Together they proposed a special surface wave propagation mode used to design nonreciprocal waveguides.

According to the researchers, the propagation behavior of the surface waves can be dynamically changed by changing the strength of the magnetic field. Relevant results were published on Laser & Photonics Reviews on Feb. 25, 2021. 

Topological photonics is a frontier scientific research field. A typical feature of topological systems is the existence of surface states without dissipation modes. This surface state can effectively bypass obstacles on the transmission path and reduce energy loss.

Current research objects are mainly concentrated on the surface state between topological non-trivial materials and air. This work mainly focuses on the surface states between two topological non-trivial systems, the results show a special pattern of electromagnetic waves, which can only propagate in one direction in 3D space. 

The researchers used metamaterials to construct time symmetry breaking and spatial symmetry breaking systems, specifically magnetized plasma and chiral hyperbolic metamaterials. The Hamiltonian, energy band structure, Berry curvature, Chen number and Fermi arc distribution of the two systems are calculated using effective parameters.

Calculation results show that although the two systems can have similar Fermi arcs, their topological properties are completely different, the surface states between them are unique. Unlike the general form of surface state propagation in three-dimensional space, which can flow in both directions, here, both surface states are restricted to half of K-space, and the direction of energy flow is unidirectional. 

Based on this property, devices such as nonreciprocal waveguides, surface wave multiplexing and division, optical switches, and optical isolation can be constructed. With reasonable parameter settings, the researchers made the propagation paths of the two surface states coincide, the tunability of magnetic field in the magnetized plasma enables the conversion between two modes.

Using magnetic field to control electromagnetic wave transmission path and achieve arbitrary splitting ratio is a step forward for the application of dynamic topological surface states. 

This work was supported by the National Natural Science Foundation of China, the Key Research Program of Frontier Science of the Chinese Academy of Sciences, and the China Scholarship Council. 

Fig. 1. The distribution of Fermi surfaces and Fermi arcs between different systems and the corresponding surface state propagation diagram. (Image by SIOM)

Fig. 2. (a) The multiplexing and division of surface states, (b-f) simulation results of dynamically routing topological surface waves. (Image by SIOM)