  Metasurfaces with strongly anisotropic optical properties can support deep
subwavelength-scale confined electromagnetic waves (polaritons) that promise
opportunities for controlling light in photonic and optoelectronic
applications. We develop a mid-infrared hyperbolic metasurface by
nanostructuring a thin layer of hexagonal boron nitride supporting deep
subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic
dispersion. By applying an infrared nanoimaging technique, we visualize the
concave (anomalous) wavefronts of a diverging polariton beam, which represent a
landmark feature of hyperbolic polaritons. The results illustrate how
near-field microscopy can be applied to reveal the exotic wavefronts of
polaritons in anisotropic materials, and demonstrate that nanostructured van
der Waals materials can form a highly variable and compact platform for
hyperbolic infrared metasurface devices and circuits.
