  The in-plane negative refraction of high-momentum (i.e., high-k) photonic
modes could enable many applications such as imaging and hyperlensing in a
planar platform at deep-subwavelength scales. However, its practical
implementation in experiments remains elusive so far. Here we propose a class
of hyperbolic metasurfaces, which is characterized by an anisotropic magnetic
sheet conductivity and can support the in-plane ultrahigh-k magnetic designer
polaritons. Based on such metasurfaces, we report the first experimental
observation of the all-angle negative refraction of designer polaritons at
extremely deep-subwavelength scales. Moreover, we directly visualize the
designer polaritons with hyperbolic dispersions. Importantly, for these
hyperbolic polaritons, we find that their squeezing factor is ultra-large and,
to be specific, it can be up to 129 in the experiments, a record-breaking value
exceeding those in naturally hyperbolic materials. The present scheme for the
achievement of negative refraction is also applicable to other natural
materials and may enable intriguing applications in nanophotonics. Besides, the
proposed metasurfaces are readily tailorable in space and frequency, which
could serve as a versatile platform to explore the extremely high confinement
and unusual propagation of hyperbolic polaritons.
