  We demonstrate that unidirectional and backscattering immune propagation of
terahertz optical waves can be achieved in a topological valley-Hall waveguide
made of graphene nanohole plasmonic crystals. In order to gain deeper physical
insights into these phenomena, the band diagram of graphene nanohole plamsonic
crystals has been investigated and optimized. We found that a graphene
plasmonic crystal with nanohole arrays belonging to the $C_{6v}$ symmetry group
possesses gapless Dirac cones, which can be gapped out by introducing extra
nanoholes such that the symmetry point group of the system is reduced from
$C_{6v}$ to $C_{3v}$. Taking advantage of this feature, we design a mirror
symmetric domain-wall interface by placing together two optimized graphene
plasmonic crystals so as to construct valley-polarized topological interface
modes inside the opened bandgap. Our computational analysis shows that the
valley-Hall topological domain-wall interface modes can be achieved at an
extremely deep subwavelength scale, and do not rely on the application of
external static magnetic fields. This work may pave a new way to develop
highly-integrated and robust terahertz plasmonic waveguides at
deep-subwavelength scale.
