PIONEERING EXPERIMENT
COHERENT emission of light
COHERENT EMISSION OF LIGHT BY THERMAL SOURCES
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Jean-Jacques GREFFET * Université Paris-Saclay, Institut d ' Optique Graduate School, CNRS, Laboratoire Charles Fabry, Palaiseau, France * jean-jacques. greffet @ institutoptique. fr
https:// doi. org / 10.1051 / photon / 202613730
This is an Open Access article distributed under the terms of the Creative Commons Attribution License( https:// creativecommons. org / licenses / by / 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
While the light emitted by an incandescent body is generally quasi-isotropic, it was demonstrated in 2002 that a diffraction grating etched onto a silicon carbide( SiC) surface can exhibit highly directional emission, comparable to that of antennas operating in the radio domain. Directivity is the signature of the existence of spatial coherence of the field in the plane of the source, an unexpected property for a thermal source. This article describes the development of the ideas that led to this experiment and its current implications. years ago " Coherent thermal emission " was an
25 unexpected title as thermal radiation was usually taken as the typical example of incoherent light. In very simple terms, light with a narrow frequency spectrum is said to be temporally coherent and light with a narrow spatial frequency spectrum is said to be spatially coherent so that blackbody radiation has a low coherence whereas laser light is highly coherent. Nonetheless, it is now possible to engineer coherent light emission by tailoring hot bodies. Thermal metasurfaces consisting of hot nanostructured surfaces is today an active field of research and most features of the emission can be controlled including the angular emission pattern( spatial coherence), the emission spectrum( temporal coherence) and the polarization.
A tutorial discussion can be found in ref. [ 1 ] and a recent rewiew [ 2 ] lists more than 500 references. The field was initiated in 1976 by the first experimental observation of a highly directional total absorption of visible light by a metallic grating [ 3 ]. Maystre and Hutley demonstrated that by ruling a shallow diffraction grating on a gold surface, a mirror could become totally absorbing for a particular angle and frequency. This highly directional absorption had been predicted theoretically and attributed to the resonant excitation of a surface plasmon. According to Kirchhoff ' s law derived in 1860 [ 4 ], emission is proportional to the absorptivity. Hence, if Kirchhoff law is valid, the thermal emission by a gold grating could be highly directional and behave as a coherent antenna. Hence, one had to conclude that either thermal emission can be coherent or that Kirchhoff ' s law is not valid, at least for gold gratings. As Kirchhoff ' s law had been derived in the framework of optical geometry which is not valid for gratings with periods on the order of the wavelength, it seemed reasonable to question Kirchhoff ' s law validity.
The experiment
The experiment that led to the observation of directional emission was triggered by Michel Olivier at CEA- Grenoble. He envisioned applications and proposed to use a grating ruled on a SiC sample. SiC is a material that supports surface phonon polaritons( SPhPs) which can mimick surface plasmons. When ruling a grating on the surface, an incident plane wave can be totally absorbed in the
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