  Negative refraction of highly squeezed polaritons is a fundamental building
block for nanophotonics, since it can enable many unique applications, such as
deep-subwavelength imaging. However, the phenomenon of all-angle negative
refraction of highly squeezed polaritons, such as graphene plasmons with their
wavelength squeezed by a factor over 100 compared to free-space photons, was
reported to work only within a narrow bandwidth (<1 THz). Demonstrating this
phenomenon within a broad frequency range remains a challenge that is highly
sought after due to its importance for the manipulation of light at the extreme
nanoscale. Here we show the broadband all-angle negative refraction of highly
squeezed hyperbolic graphene plasmons in the infrared regime, by utilizing the
nanostructured graphene metasurfaces. The working bandwidth can vary from
several tens of THz to over a hundred of THz by tuning the chemical potential
of graphene.
