Near-Field Thermal Radiation between Metasurfaces

Evanescent waves induced by thermal fluctuations can tunnel through nanoscale gap spacing, leading to super-Planckian thermal radiation. However, investigations of near-field thermal radiation of macroscopic objects have been limited to simple planar or effectively planar geometries until recently. Based on exact formulations including the scattering theory and Green's function method, patterning thin films into ID and 2D metasurfaces is found to increase the radiative heat flux by more than 1 order of magnitude in a certain range of thicknesses. The underlying mechanism of this counterintuitive phenomenon lies in the excitation of hyperbolic modes supporting high local density of states for broad frequency and k-space regimes. The radiative heat flux of a 2D metasurface increases monotonically with the thickness, while the heat flux of a ID metasurface is not so sensitive to the thickness and is surprisingly higher than that of its 2D counterparts. The stark difference is attributed to the rapid-decay surface modes supported by a ID metasurface in addition to the slow-decay hyperbolic modes.