Thermal radiation is usually considered an incoherent signal because it results from the random fluctuations of materials. Circular polarization in the heat released by most typical thermal emitters is weak to zero circular polarization.

Surprisingly, thermal radiation from numerous celestial bodies reaching Earth shows strong circular polarization. This fascinating phenomenon explains mysteries about the early cosmos, reveals powerful magnetic fields in some condensed stars and even gives a possible hint of life.

Using artificially textured surfaces known as metasurfaces, researchers at Purdue University have made significant advances in thermal radiation. They have developed a brand new technique to generate rotating heat radiation in a regulated and effective way.

This work offers a new way to excite this type of radiation, which has the potential to be used in a variety of applications, including thermal imaging and communications.

The scientists found they could produce mostly left-handed circularly polarized thermal radiation in all directions by using a metasurface consisting of a series of F-shaped structures, creating a non-vanishing optical helicity for the first time. The team achieved 39% of the fundamental limit of optical helicity with their design. They also proved that the symmetries of the metasurface can be used to modify the properties of the emitted thermal photons, proving that thermal radiation can be effectively controlled.

 unique spider textures
The unique spin textures of the engineered thermal emission of our metasurfaces can be exploited as high-contrast infrared beacons in outdoor environments, as the background thermal emission from other natural objects is highly disjointed without any spin textures.

Dr. Zubin Jacob, Elmore Associate Professor of Electrical and Computer Engineering, said: “This research may have important implications for understanding the ubiquitous phenomenon of thermal radiation and for developing new technologies.”

“Potential applications include using the metasurface as a wide-angle, narrow-band circularly polarized mid-infrared light source for optical gas detection and infrared imaging. In addition, the unique spectral, spatial and spin function of the designed thermal emission can be exploited as passive infrared beacons in outdoor environments, making them useful in remote sensing technology.

Ph.D. student Xueji Wang said: “We are extremely excited about the potential of this discovery. It not only deepens our understanding of thermal radiation, but also opens up new possibilities for technological advancements in several areas.”

Magazine reference:

  1. Xueji Wang et al., Observation of non-vanishing optical helicity in thermal radiation from symmetry-broken metasurfaces, Science Advances (2023). DOI: 10.1126/sciadv.ade4203