Pseudo-spin\u2013valley coupled edge states in a photonic topological insulator

Yuhao Kang,Azriel Z Genack,Xiaojun Cheng,Xiang Ni,Alexander B Khanikaev

doi:10.1038/s41467-018-05408-w

Abstract

Pseudo-spin and valley degrees of freedom engineered in photonic analogues of topological insulators provide potential approaches to optical encoding and robust signal transport. Here we observe a ballistic edge state whose spin–valley indices are locked to the direction of propagation along the interface between a valley photonic crystal and a metacrystal emulating the quantum spin–Hall effect. We demonstrate the inhibition of inter-valley scattering at a Y-junction formed at the interfaces between photonic topological insulators carrying different spin–valley Chern numbers. These results open up the possibility of using the valley degree of freedom to control the flow of optical signals in 2D structures.

Highlights

Pseudo-spin and valley degrees of freedom engineered in photonic analogues of topological insulators provide potential approaches to optical encoding and robust signal transport
There has been growing interest in simpler designs in which topological states could be emulated in photonics by using lattice symmetries and associated synthetic degree of freedom (DOF) that might be achieved in structures that are less challenging to fabricate
The development of photonic topological insulators (TIs) opens up the possibility of extending valleytronics to optics where the valley DOF is combined with the pseudo-spin DOF responsible for the topological order

Summary

Results

The triangular unit cell differs from a trivial photonic crystal in that it breaks inversion symmetry. This gives rise to different local Chern numbers in the two valley sectors. The Chern number difference is (−0.5)−0.5 = −1 for the super-cell shown in Fig. 2a in the spin-down sector of the K valley. This corresponds to a backwards propagating spin-down state at the K valley. Measurements are performed for a zigzag cut of the crystal, as shown, because reflection of the edge state at the interface between the metawaveguides and air is inhibited in this case[10]. The source and detector dipoles are inserted vertically norm(E) (V/m) Average dwell time (ns) a

Output d6

Discussion

Methods