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Broadband directional control of thermal emission

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Science  23 Apr 2021:
Vol. 372, Issue 6540, pp. 393-397
DOI: 10.1126/science.abc5381

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Broadband thermal beaming

Thermal radiation emits over a wide range of wavelengths and over a wide range of angles. Xu et al. constructed a material that allows a range of wavelengths to emit over a much narrower range of angles. This property allowed the authors to beam thermal energy preferentially in one direction. The strategy requires carefully exploiting stacks of epsilon-near-zero films in which the angular range of thermal emission is controlled by the film thickness. This design could be useful in thermal camouflage and passive radiative cooling applications.

Science, this issue p. 393

Abstract

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.

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