  The nature of light-matter interaction is governed by the spatial-temporal
structures of a light field and material wavefunctions. The emergence of the
light beam with transverse phase vortex, or equivalently orbital angular
momentum (OAM) has been providing intriguing possibilities to induce
unconventional optical transitions beyond the framework of the electric dipole
interaction. The uniqueness stems from the OAM transfer from light to material,
as demonstrated using the bound electron of a single trapped ion. However, many
aspects of the vortex light-matter interaction are still unexplored especially
in solids with extended electronic states. Here, we unambiguously visualized
dipole-forbidden multipolar excitations in a solid-state electron system; spoof
localized surface plasmon, selectively induced by the terahertz vortex beam.
The results obey the selection rules governed by the conservation of the total
angular momentum, which is numerically confirmed by the electromagnetic field
analysis. Our results show light's OAM can be efficiently transferred to an
elementary excitation in solids.
