Helicity-selective and spectrally tunable chiral thermal emissions.

Integrating nonlocal metasurfaces with thermal photonics has endowed thermal emissions with temporal and spatial coherence, as well as chirality. However, the static response of current thermal emitters hinders their broader applications, especially in high-precision sensing. Here, we present an anisotropic thermal metasurface that harnesses exclusively elevated temperatures inherent in thermal photonics to achieve helicity-switchable and wavelength-tunable circularly polarized coherent thermal emissions in the mid-infrared. Through a meticulous design to achieve a pair of high-Q quasi-guided mode resonances with opposite chirality and significant emission circular dichroism, we experimentally demonstrate helicity-switchable circularly polarized thermal emissions with high temporal coherence (Q > 150) and strong emission circular dichroism (>0.8) over a ~ 100 nm wavelength range through a temperature change of 250 K. We further reveal that simple geometric design enables full polarization tailoring. Our platform offers a compact and scalable pathway toward on-chip applications including circular dichroism spectroscopy for enantiomer identification.