  Optical metasurfaces have great potential to form the platform for
manipulation of surface waves. A plethora of advanced surface-wave phenomena
utilizing negative refraction, self-collimation and channeling of 2D waves can
be realized through on-demand engineering of dispersion properties of a
periodic metasurface. In this letter, we report on the first-time direct
experimental polarization-resolved measurement of dispersion of 2D optical
plasmons supported by an anisotropic metasurface. We demonstrate that a
subdiffractive array of strongly coupled resonant anisotropic plasmonic
nanoparticles supports unusual optical surface waves with mixed TE- and TM-like
polarizations. With the assistance of numerical simulations we identify dipole
and quadrupole dispersion bands. The shape of isofrequency contours changes
drastically with frequency exhibiting nontrivial transformations of their
curvature and topology that is consistently confirmed by the experimental data.
By revealing polarization degree of freedom for surface waves, our results open
new routes for designing of planar on-chip devices for surface photonics.
