Near-field optical characterization of atomic structures and polaritons in twisted two-dimensional materials

Polaritons are quasiparticles generated from strong interaction between a photon and an electric or magnetic dipole-carrying excitation. They can confine light into a small space that is beyond the diffraction limit of light, thus have greatly advanced the development of nano photonics, nonlinear optics, quantum optics and other related research. Van der Waals two-dimensional (2D) crystals provide an ideal platform for studying nano-polaritons due to reduced material dimensionality. In particular, stacking and twisting offer additional degree of freedom for manipulating polaritons that are not available in a single layer material. In this paper, we review near-field optical characterization of various structures and polaritonic properties of stacked/twisted 2D crystals reported recent years, including domain structures of stacked few-layer graphene, moiré superlattice structures of twisted 2D crystals, twisted topological polaritons, twisted chiral plasmons, etc. We also propose several exciting directions for future study of polaritons in stacked/twisted 2D crystals.