Non-Vanishing Optical Helicity in Thermal Radiation with Symmetry-Broken Metasurfaces

Thermal radiation describes the universal phenomenon that all objects at non-zero temperatures emit infrared electromagnetic energy. Significant research progress has been made so far in tailoring its temporal coherence and spatial coherence. However, the photon spin, another crucial characteristic of electromagnetic radiation, is commonly ignored, since most thermal emitters show weak to zero spin angular momentum (SAM) in the emitted waves. Surprisingly, the thermal radiation reaching the earth from many astronomical objects possesses significant circular polarization. The unique phenomenon provides strong evidence for the presence of a magnetic field around stars or reveals the existence of chiral organic molecules. Therefore, revealing photon spin characteristics in thermal radiation is of fundamental interest as it contains unique information regarding the emitters. Here, we demonstrate spinning thermal radiation with a non-vanishing optical helicity via symmetry-broken metasurfaces. We design non-vanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation, where our design reaches 39% of the fundamental limit. Our results firmly suggest metasurfaces can impart spin coherence in the incoherent radiation excited by thermal fluctuations. The symmetry-based design strategy also provides a general pathway for comprehensively controlling thermal radiation in its temporal, spatial, and spin coherence.