Circularly polarized thermal emission driven by chiral flatbands in monoclinic metasurfaces.

Achieving circularly polarized thermal emissions with high spatiotemporal coherence using planar structures has long been considered to be elusive. Here, we use nonlocal metasurfaces with monoclinic lattices that break mirror symmetry to efficiently achieve circularly polarized thermal emissions with both high temporal and spatial coherence. We design a chiral metasurface based on waveguide arrays with periodically shifted segments that have a saddle-shaped chiral and high- dispersion band. The parabolic shape along one direction ensures minimum involvement of spatial/Fourier components at each frequency, thereby achieving circularly polarized thermal emission with high spatial coherence. Meanwhile, the flatband behavior along the other direction allows the use of a slot-shaped spatial filter and a mid-infrared lens to collect emissions from large-area metasurfaces, thus improving power collection efficiency without affecting temporal coherence. Our experiments demonstrate circularly polarized thermal emissions with high temporal coherence ( > 200) and very large circular dichroism (~0.8).