Flatland Optics with Hyperbolic Metasurfaces

In this Perspective, we discuss the physics and potential applications of planar hyperbolic metasurfaces (MTSs), with emphasis on their in-plane and near-field responses. After revisiting the governing dispersion relation and properties of the supported surface plasmon polaritons (SPPs), we discuss the different topologies that uniaxial MTSs can implement. Particular attention is devoted to the hyperbolic regime, which exhibits unusual features, such as an ideally infinite wave confinement and local density of states. In this context, we clarify the different physical mechanisms that limit the practical implementation of these ideal concepts using materials found in nature, and we describe several approaches to realize hyperbolic MTSs, ranging from the use of novel 2D materials such as black phosphorus to artificial nanostructured composites made of graphene or silver. Some exciting phenomena and applications are then presented and discussed, including negative refraction and the routing of SPPs within the surface, planar hyperlensing, dramatic enhancement and tailoring of the local density of states, and broadband super-Planckian thermal emission. We conclude by outlining our vision for the future of uniaxial MTSs and their potential impact for the development of nanophotonics, on-chip networks, sensing, imaging, and communication systems.