In-plane hyperbolic phonon polaritons: materials, properties, and nanophotonic devices

In-plane hyperbolic phonon polaritons (HPhPs) are phonon-mediated hybrid electromagnetic modes, particularly in two-dimensional (2D) van der Waals (vdW) crystals, which have attracted increasing attention because of their peculiar optical properties and promising nanophotonic applications. Here, we review the most recent advances in in-plane HPhPs in terms of materials, optical properties and nanophotonic devices. We begin with a survey of recently discovered in-plane anisotropic vdW materials and bulk crystals that naturally exhibit in-plane HPhPs. The fundamental properties of HPhPs in these anisotropic materials are then discussed, focusing on propagation directionality such as direction rotation, unidirectional excitation, canalization, negative reflection, and negative refraction. Finally, we discuss the present applications of in-plane HPhPs in nanophotonic devices and offer a perspective on future developments of in-plane HPhPs towards nanophotonic chips.