Figure 2: Concept and implementation of the tungsten/hafnium dioxide refractory metamaterial.

(a) Calculated absorptivity characteristic of the metamaterial at normal incidence. At vacuum wavelengths below the topological transition the medium supports radiative modes resulting in high absorptivity. Beyond the transition the metamaterial allows only modes with large tangential components of the wavevector which cannot couple to optical modes propagating in vacuum. This leads to a strong suppression of the structure’s absorptivity and thus of its emissivity. Because of the small unit cell size excellent agreement is seen between the effective medium and rigorous transfer matrix theory. (b) Comparison of the theoretically designed, based on permittivity data provided by Roberts⁵², and ellipsometrically extracted relative permittivity parameters for the metamaterial structure (see methods). In all simulations the hafnium dioxide layers are assumed to be lossless and dispersionless with a relative permittivity of ɛ=3.88. (c) Schematic image of the refractory metamaterial design. The dashed box shows the metamaterial unit cell. (d) SEM image of the fabricated refractory metamaterial. By choosing the thicknesses of the nano-structured refractory metal and oxidic dielectric layers, topological transitions can be tuned throughout the infrared.