Broadband Unidirectional Thermal Emission Enhancement of Phase-Gradient Metasurface Based on Surface Plasmon Polaritons

Abstract

Compared with external coherent light manipulation, the realization of unilateral-oriented thermal emission is more challenging. Although a specific metasurface is confirmed of the angular selectivity, the emissivity is generally mirror-symmetric along the surface normal for uncorrelated phases between different positions of the thermal source. Here, from the metallic patch array conditions derived for unilateral thermal emission, a phase-gradient metasurface in combination with surface plasmon polaritons is demonstrated which exhibit wide-angled unilateral enhancements of thermal emission while the radiations in the symmetric directions are suppressed. By directly heating the device to 120 °C, the acquired spatial thermal emission spectrums indicate that at a wavelength of λ = 11.5 µm, the changed emissivity 0.61–0.46 with the orientation angle from 10° to 40° is asymmetric to 0.32–0.21 from −10° to −40°. Dealing with the phase response of meta-atoms, the proposed method and phase-modulated structure are helpful for the direction of controllable thermal emission.