Chapter 3. Anisotropic Polaritons in Layered Two-dimensional (2D) Materials

Perhaps the most significant progress in the field of infrared optics and nanophotonics has been made through the real space realisation of polaritons in two-dimensional materials that provide maximum light confinement functionalities. The recent breakthrough discovery of in-plane hyperbolicity in natural van der Waals materials has revealed a most exciting optical property that enables in-plane anisotropic dispersion. Yet, the most intriguing feature of in-plane anisotropic dispersion is the manipulation of polaritons at the nanoscale. This development has opened a new window of opportunity in order to develop unique nanophotonic devices with unprecedented control. This chapter will cover these developments with focus on fundamental understandings and progress of real space visualisation of in-plane anisotropic polaritons in the near-field range. The last section will conclude with the future prospects of this rapidly emerging area.