Highly circularly polarized and coherent thermal emission based on chiral quasi bound states in the continuum.

Polarization, temporal coherence, and spatial coherence are crucial for far-field thermal emission. However, achieving chiral thermal emission with both ultra-narrow bandwidth and ultrahigh directionality remains a challenge. In this study, we address this problem by combining the principles of band folding and chiral quasi bound states in the continuum. The demonstrated thermal emitter, a tri-layered structure consisting of a planar chiral silicon metasurface, a silica spacer, and a reflecting gold film, numerically achieves an emissivity circular dichroism of 0.984, a full width at half maximum of 1.6 nm, and a divergence angle of 1° at wavelength 1170 nm, surpassing the state-of-the-art thermal emitters. Our finding provides a new, to our knowledge, approach for designing chiral thermal emitters, which may find use in the areas of thermal lighting, infrared camouflage, thermal imaging, and infrared sensing.