Unidirectional propagation of helical edge states via exciting pseudospin d states in two-dimensional photonic crystals

Abstract

Helical edge states (HESs) in two-dimensional topological spin photonic crystals can be used to realize pseudospin-locked unidirectional propagation of waves. In general, the excitation of HESs is by taking light sources carrying orbital angular momentum of order 1 to stimulate the pseudospin p states in spin photonic crystals. In this paper, we think of the HESs as combinations of the pseudospin p states and the pseudospin d states, which corresponding to the pseudospin modes carrying orbital angular momentum of order 1 and the pseudospin modes carrying orbital angular momentum of order 2, respectively, with their chirality related to the unidirectional propagations of the HESs. By analyzing the field distributions of HESs and via exciting the pseudospin d states with light sources carrying orbital angular momentum of order 2, we demonstrate the unidirectional propagation of the HESs in spin photonic crystals and verify their robustness by checking the unidirectional performance as they propagate along a Z-shape interface containing sharp corners, cavity defect, and disorders. Our study deepens understanding of the nature of HESs and expands methods to excitation and regulation of HESs in topological spin photonics.