Direct observation of plasmonic index ellipsoids on a deep-subwavelength metallic grating

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 homoge-neous uniaxial form-birefringent metal, exhibiting different optical responses for polarizations along dif-ferent optical axes. Therefore, this form-birefringent metal supports anisotropic surface plasmon polaritons that are characterized by directly imaging the generated plasmonic index ellipsoids in reci-procal space. The observed plasmonic index ellipsoids also show a rainbow effect, where different colors are dispersively distributed in reciprocal space. © 2011 Optical Society of America OCIS codes: 240.6680, 310.6628, 050.2555. 1