  Hyperbolic polaritons in van der Waals materials and metamaterial
heterostructures provide unprecedented control over light-matter interaction at
the extreme nanoscale. Here, we propose a concept of type-I hyperbolic
metasurface supporting highly-squeezed magnetic designer polaritons, which act
as magnetic analogues to hyperbolic polaritons in the hexagonal boron nitride
(h-BN) in the first Reststrahlen band. Comparing with the natural h-BN, the
size and spacing of the metasurface unit cell can be readily scaled up (or
down), allowing for manipulating designer polaritons in frequency and in space
at will. Experimental measurements display the cone-like hyperbolic dispersion
in the momentum space, associating with an effective refractive index up to 60
and a group velocity down to 1/400 of the light speed in vacuum. By tailoring
the proposed metasurface, we experimentally demonstrate an ultra-compact (with
a footprint shrunken by 3600 times) integrated designer polariton circuit
including high-transmission 90{\deg} sharp bending waveguides and waveguide
splitters. The designed metasurface with a low profile, lightweight, and ease
of access, can serve as an alternatively promising platform for emerging
polaritonics, and may find many other potential applications, such as
waveguiding, sensing, subdiffraction focusing/imaging, low-threshold Cherenkov
radiation, strong magnetic transition enhancement, wireless energy transfer,
and so forth.
