Polar metamaterials with graded air inclusions for infrared radiation managing

Polar materials, i.e. those materials where it is possible to excite the collective oscillations of the lattice ions, along with derived phononic structures offer the possibility of manipulating and amplificating IR emissivity by the excitation of such surface modes. Since the oscillation frequencies of the crystalline lattice ions are typically lower, compared to the plasma frequencies, they fall in the infrared wavelength range. The asymmetric spectral emissivity in the infrared range of a metamateriale composed by subwavelength oriented air inclusions into a polar matrix was investigated. We show hoe the longitudinal phononic resonance can be excited and tuned as function of the inclusions content and orientation, within the application limit of the effective medium homogenization technique. Furthermore, our numerical result show that it is possible to enhance the emissivity in a given direction rather than in the opposite one by continuously grading the inclusions content along the matrix thickness. As a possible application we designed a graded air inclusion pattern into a silicon carbide layer, only few microns thick. A strongly asymmetry between forward and backward emission along the normal direction to the surface is obtained.