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Super-Planckian emission cannot really be ‘thermal’

A heat-powered emitter can sometimes exceed the Planck thermal-emission limit. We clarify when such super-Planckian emission is possible, arguing that far-field super-Planckian emission requires a distribution of energy that is not consistent with a unique temperature, and therefore the process should not be called ‘thermal emission’.

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Fig. 1: Thought experiment that explains Kirchhoff’s law of thermal radiation.
Fig. 2: Subwavelength thermal emitters.
Fig. 3: Non-equilibrium heat-powered emission.


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M.A.K. and Y.X. acknowledge support from the National Science Foundation (NSF) (grant no. 1750341) and the Office of Naval Research (N00014-20-1-2297). M.S. acknowledges support from the NSF (grant no. 2108288) and the Welch Foundation (A-1886). We thank D. Seletskiy, whose live session at the SPIE Digital Forum helped us to crystallize some key questions, and J. Choy, for critical reading of the manuscript.

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Correspondence to Mikhail A. Kats.

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Nature Photonics thanks Takashi Asano, Jacob Khurgin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Supplementary information

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Supplementary Figures 1-5, Supplementary Discussion

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Source Data Fig. 1

Fig. 1b source data.

Source Data Fig. 2

Figs. 2b and 2d source data.

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Figs. 3d and 3e source data.

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Xiao, Y., Sheldon, M. & Kats, M.A. Super-Planckian emission cannot really be ‘thermal’. Nat. Photon. 16, 397–401 (2022).

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