Functional-Graded Index Metasurfaces for Infrared Radiation and Guiding

We present the fundamental principle of plasmonic-graded-index materials for radiating and guiding at infrared range. The method to derive effective refractive index is presented and effective refractive index engineering is comprehensively investigated by obtaining complex dispersion diagrams. As an illustrative example for radiating structure, a graded index metasurface is designed for collimating the power emanating from an aperture to a steered narrow beam. The center frequency of operation is 57.5 THz. The half power beam widths of the beam radiated from a 15λ × 15λ metasurface to θ = 30° direction are 6° and 16° in elevation and azimuth planes, respectively. Radiation efficiency is calculated to be 86%. To demonstrate the guiding application, a flat step index waveguide is designed. The surface wave guiding is characterized by obtaining complex dispersion diagram and mode profiles. The applications of such waveguides in feeding antennas and routing surface waves are demonstrated with some unique designs.