Negative refraction of ultra-squeezed in-plane hyperbolic designer polaritons

The in-plane negative refraction of high-momentum(i.e.,high-k)photonic modes could enable many applications such as imaging,focusing,and waveguiding 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 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.To be specific,it can be up to 129 in the experiments,an ultra-high value exceeding those in naturally hyperbolic materials.This work may pave a way toward exploring the extremely high confinement and unusual propagation of magnetic designer polaritons over monolayer or twisted bilayer hyperbolic metasurfaces.