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Observation of Strong Nonreciprocal Thermal Emission

Zhenong Zhang ^*, Alireza Kalantari Dehaghi ^*, Pramit Ghosh , and Linxiao Zhu ^†

Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

^*These authors contributed equally to this work.
^†Contact author: lqz5242@psu.edu

Phys. Rev. Lett. 135, 016901 – Published 30 June, 2025

DOI: https://doi.org/10.1103/PhysRevLett.135.016901

Abstract

The Kirchhoff’s law of thermal radiation stating the equivalence of emissivity and absorptivity at the same wavelength, angle, and polarization, has completely constrained emission and absorption processes. Achieving strong nonreciprocal emission points to fundamental advances for applications such as energy harvesting, heat transfer, and sensing, but strong nonreciprocal thermal emission has not been experimentally realized. Here, we observe strong nonreciprocal thermal emission using a custom-designed angle-resolved magnetic thermal emission spectroscopy and an epitaxially transferred gradient-doped metamaterial. We show that under magnetic field, the metamaterial strongly breaks Kirchhoff’s law, with a difference between emissivity and absorptivity at the same wavelength and angle reaching as high as 0.43. Significant nonreciprocal emission persists over broad spectral and angular ranges. The demonstration of strong nonreciprocal thermal emission and the approach can be useful for systematic exploration of nonreciprocal thermal photonics for thermal management, infrared camouflage, and energy conversion.

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Radiation Imbalance: New Material Emits Better Than It Absorbs

Published 30 June, 2025

A newly designed structure exhibits the largest-recorded emissivity–absorptivity difference, a property that could prove useful in energy-harvesting and cloaking devices.

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