  Polar dielectrics with low crystal symmetry and sharp phonon resonances can
support hyperbolic shear polaritons - highly confined surface modes with
frequency-dependent optical axes and asymmetric dissipation features. So far,
these modes have been observed only in bulk natural materials at mid-infrared
frequencies, with properties limited by available crystal geometries and phonon
resonance strength. Here we introduce hyperbolic shear metasurfaces: ultrathin
engineered surfaces supporting hyperbolic surface modes with symmetry-tailored
axial dispersion and loss redistribution that can maximally enhance
light-matter interactions. By engineering effective shear phenomena in these
engineered surfaces, we demonstrate geometry-controlled, ultra-confined,
low-loss hyperbolic surface waves with broadband Purcell enhancements,
applicable across a broad range of the electromagnetic spectrum.
