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Super-Planckian emission cannot really be ‘thermal’

Nature Photonics volume 16, pages 397–401 (2022)Cite this article

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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’.

The spontaneous release of thermal energy by emission of electromagnetic radiation (‘thermal radiation’ or ‘thermal emission’) is a fundamental property of matter. Together, Planck’s law and Kirchhoff’s law of thermal radiation describe the characteristic emission spectrum when an object has a well-defined internal temperature1,2. Over the past two decades, there has been a flurry of research activity to engineer the thermal emission from various materials and structures3, for example to enhance the temporal4 or spatial5 coherence of thermal emission, or to enable dynamic control6. There have also been theoretical predictions7 and experimental claims8,9 of ‘super-Planckian thermal emission’, with spectral radiance apparently exceeding the blackbody limit over some wavelength range. The possibility of super-Planckian thermal emission has substantial implications for applications that rely on radiative energy transfer, including energy harvesting10, cooling and thermoregulation11, and light sources12. However, thermal emission exceeding the blackbody limit in the far field at first seems to be in contradiction with the laws of thermodynamics, and this can lead to confusion, some of which has been discussed and clarified in the literature13,14. In this Commentary, we aim to provide an intuitive perspective for several recent developments in this research area, further clarifying how and when super-Planckian emission can indeed be achieved without violating the laws of thermodynamics, and why it is helpful to consider the phenomenon as part of a broader class of heat-powered emission, as opposed to super-Planckian ‘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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Acknowledgements

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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Authors and Affiliations

  1. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI, USA

    Yuzhe Xiao & Mikhail A. Kats

  2. Department of Chemistry, Texas A&M University, College Station, TX, USA

    Matthew Sheldon

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  1. Yuzhe Xiao
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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

Supplementary Information

Supplementary Figures 1-5, Supplementary Discussion

Source data

Source Data Fig. 1

Fig. 1b source data.

Source Data Fig. 2

Figs. 2b and 2d source data.

Source Data Fig. 3

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). https://doi.org/10.1038/s41566-022-01005-y

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