Functional Mid-Infrared Polaritonics in van der Waals Crystals

Despite high scientific and technological potential, nanophotonics research in the mid-infrared regime remains relatively less explored compared to other frequency bands, largely because the mid-infrared requires a totally different set of optical materials. Polaritons in layered 2D materials, or van der Waals (vdW) crystals, provide a new set of building blocks for mid-infrared nanophotonics. Herein, the recently reported polaritonic properties of various vdW crystals are summarized in the context of their mid-infrared applications. Both polaritons in vdW crystals with naturally anisotropic atomic structures as well as tunable plasmon polaritons in vdW semimetals are discussed. The extreme field confinement of 2D polaritons (confinement factors approximate to 100) enhances both their radiative heat transfer as well as the optical coupling to the vibrational modes of molecules, allowing for highly sensitive chemical detection. Their tunable properties, meanwhile, enable dynamic modulation of mid-infrared light to realize dynamic phase shifters, mid-infrared modulators, and spectrally tunable thermal emitters. By vertically stacking vdW crystals, it is possible to create a large variety of new effective materials with substantially different polaritonic properties compared to their building blocks.