Fig. 1 Two-dimensional deep subwavelength grating constructing a birefringent metal with different epsilon at different optical axes.
"Form-birefringence", artificial metamaterials first proposed by our group, consist of two or more materials in a deep subwavelength scale, and act as anisotropic crystals in electrostatic field approximation with averaging space-variant polarizabilities of isotropic dielectrics. Recently, we constructed a uniaxial form birefringent metal (Figs. 1 and 2) that exhibits different dielectric polarizabilities along different optical axes as well as its supported optical anisotropy of surface plasmon polariton waves. The generated plasmonic index ellipsoid that exists in k space has been directly mapped and characterized in our experiment (Fig. 3). The discovery of this anisotropic plasmonic metamaterial further completes analogy between artificial plasmonic metamaterials and conventional optical crystals, thereby providing opportunities to miniaturize myriad existing optical devices on-a-chip with plasmonics into nanometers scale (e.g. in-plane polarizer and phase compensator for surface plasmon waves).
Fig. 2 SEM image of the fabricated structure.
The concept of "form birefringent" metal is not limited within the specific case shown above. The extended 3D case can be designed more complicated to adapt more complex Bravais crystal lattices and thus realize a bi or multi-axial anisotropic metal. Beyond the analog of nature crystals, this concept of engineering metal in a deep subwavelength scale can be applied to design and manufacture inhomogeneous metals with space-variant local polarizabilities.
Fig. 2 Plasmonic index ellipsoid on the form birefringent metal directly captured in k space showing anisotropic surface plasmon excitations.
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