Analog Quantum Valley-Hall and Quantum Spin Hall Plasmons in Graphene Metasurfaces with Low Point Group Symmetry

In recent years, topological physics of classical waves in artificial crystals has become an emerging field of research. While Dirac cones and valley-related physics are conventionally studied in these systems with C6v and C3v point-group symmetries, here analog quantum valley Hall and quantum spin Hall plasmons in graphene metasurfaces with lower point-group symmetries are explored. First, it is shown that a single-layer graphene sheet with rectangle holes respecting to the C2v point group symmetry can host a mirror (sigma v) symmetry-protected Dirac cone along the X-M edge of the Brillouin zone. Then we demonstrate that introducing further circular holes to the graphene sheet can break the mirror symmetry (i.e., reducing C2v symmetry to C1v) and thus gap out the Dirac cone, which allows us to explore the valley and layer-pseudospin related topological plasmons in these graphene metasurfaces with low point group symmetry. Valley-locking unidirectional propagations along the domain-wall interface of a single-layer graphene metasurface and layer-pseudospin converter in a double-layer graphene metasurface are explicitly demonstrated for graphene plasmons in the THz range. This work provides a new design principle for exploring Dirac cones, valley, and pseudospin related physics using much lower point-group symmetries. Most current studies of analog quantum valley and spin Hall effects in topological photonics are investigated based on high point-group symmetries. The possibility of realizing these physics in systems with much lower point group symmetry remains rarely unexplored. This work provides a new design principle for exploring valley and pseudospin related physics in systems with low point-group symmetries. image