  A hyperbolic plasmonic surface supports highly directional propagating
polaritons with extremely large density of states. Such plasmon polaritons have
been realized in artificially structured metasurfaces. However, the upper bound
of the achievable plasmon wave vector is limited by the structure size, which
calls for a natural hyperbolic surface without any structuring. Here, we
experimentally demonstrate a natural hyperbolic plasmonic surface based on thin
films of WTe2 in the light wavelength range of 16 to 23 microns by far infrared
absorption spectroscopy. The topological transition from the elliptic to the
hyperbolic regime is further manifested by mapping the iso-frequency contours
of the plasmon. Moreover, the anisotropy character and plasmon frequency
exhibit prominent temperature dependence. Our study demonstrates the first
natural platform to host 2D hyperbolic plasmons, which opens exotic avenues for
the manipulation of plasmon propagation, light-matter interaction and light
emission in planar photonics.
