We constructed a metallic grating on a deep-subwavelength scale and tested its plasmonic features in visible frequencies. The deep-subwavelength metallic grating effectively acts as an anisotropic homogeneous uniaxial form-birefringent metal, exhibiting different optical responses for polarizations along different optical axes. Therefore, this form-birefringent metal supports anisotropic surface plasmon polaritons that are characterized by directly imaging the generated plasmonic index

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... s in reciprocal space. The observed plasmonic index ellipsoids also show a rainbow effect, where different colors are dispersively distributed in reciprocal space.