Mapping and Manipulation of Topological Singularities: From Photonic Graphene to T-Graphene

Abstract

Topological singularities (TSs) in momentum space give rise to intriguing fundamental phenomena as well as unusual material properties, attracting a great deal of interest in the past decade. Recently, we demonstrated universal momentum-to-real-space mapping of TSs and pseudospin angular momentum conversion using photonic honeycomb (graphene-like) and Lieb lattices. Such mapping arises from the nontrivial Berry phase winding and is thus of topological origin. In this paper, after a brief review of previous observations of pseudospin–orbital conversion, we present the first theoretical analysis and experimental demonstration of TS mapping in a new type of Dirac-like structure, namely, the T-graphene lattice. Unlike other lattices, there are two coexisting but distinct TSs located at different high-symmetry points in the first Brillouin zone of T-graphene, which enables controlled topological charge conversion in the same lattice. We show active manipulation of the TS mapping, turning the two TSs into vortices of different helicities, or one into a high-order vortex but the other eliminated and emerged as a quadrupole. Such mapping and manipulation of TSs may find applications in optical communications and quantum information, and may bring insight into the study of other Dirac-like structures with multiple TSs beyond the 2D photonic platform.