Spatiotemporally Coherent Chiral Thermal Emission Driven by Anisotropic Diatomic Metasurfaces

ABSTRACT Simultaneously endowing planar thermal sources with exceptional spatiotemporal coherence and pronounced circular polarization remains challenging. Here, we demonstrate an anisotropic diatomic metasurface that strictly breaks in‐plane mirror symmetry to efficiently generate highly pure chiral thermal emission. Our design utilizes y ‐axis‐aligned germanium nanopillar pairs with oppositely rotated (±45°) notches, supporting a unique parabolic‐cylindrical band. Such a configuration enables orthogonal control over radiation: suppressed transverse coupling along the x ‐axis maintains a momentum‐invariant flatband, anchoring the emission frequency via a divergent density of optical states. This ensures ultranarrow emission linewidths and amplifies chiral emission through slow‐light effects. Conversely, the steep longitudinal parabolic dispersion along the y ‐axis serves as a wavevector filter, dictating high spatial coherence and fundamentally resolving the traditional efficiency‐coherence trade‐off. Optical characterizations reveal circularly polarized thermal emission featuring an ultralong spatial coherence length ( Lc ∼ 49 λ ), high temporal coherence ( Q ∼ 150), and robust circular dichroism (0.92) retained over a 12° range.