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Valley vortex states and degeneracy lifting via photonic higher-band excitation
preprintposted on 2023-11-30, 06:33 authored by Daohong Song, Daniel Leykam, Jing Su, Xiuying Liu, Liqin Tang, Sheng Liu, Jianlin Zhao, Nikolaos K. Efremidis, Jingjun Xu, Zhigang Chen
We demonstrate valley-dependent vortex generation in a photonic graphene. Without breaking the inversion symmetry, excitation of two equivalent valleys leads to formation of an optical vortex upon Bragg-reflection to the third valley, with its chirality determined by the valley degree of freedom. Vortex-antivortex pairs with valley-dependent topological charge flipping are also observed and corroborated by numerical simulations. Furthermore, we develop a three-band effective Hamiltonian model to describe the dynamics of the coupled valleys, and find that the commonly used two-band model is not sufficient to explain the observed vortex degeneracy lifting. Such valley-polarized vortex states arise from high-band excitation without inversion symmetry breaking or synthetic-field-induced gap opening. Our results from a photonic setting may provide insight for the study of valley contrasting and Berry-phase mediated topological phenomena in other systems.