  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 have 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 Berry phase encircling the Dirac or Dirac-like cones,
and is thus of topological origin. In this paper, we briefly present previous
observations of topological charge conversion, and then we present our first
theoretical analysis and experimental demonstration of TS mapping in a new
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 into a quadrupole. Such TS manipulation and pseudospin-to-orbital
conversion 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.

Optics