Multilayered Low‐Q Metasurfaces for Broadband Asymmetric Directional Thermal Emission Across the LWIR Window

Precise control over the directionality of thermal emission is crucial for applications such as thermal camouflage and radiative cooling—particularly within the 8–14 µm long‐wave infrared (LWIR) atmospheric window. Conventional thermal emitters, however, are intrinsically angularly symmetric, fundamentally limiting their performance in such scenarios. Here, we introduce a multilayer‐film metasurface emitter that delivers broadband, polarization‐robust, and asymmetric directional emission across the entire LWIR band. Our theory identifies a critical incidence angle beyond which a tailored phase gradient enables unidirectional excitation of evanescent waves, and we show that optimizing the resonator quality factor by multilayer films is essential for achieving smooth phase modulation over a broad spectral range. By arranging engineered metallic resonators in a supercell architecture, we induce strong broadband absorption—and hence enhanced thermal emission—at positive angles while suppressing it at their negative counterparts. This asymmetric emission performance, covering the entire LWIR window, achieves a 60% improvement over previous reports, establishing a new benchmark for passive broadband asymmetric thermal meta‐emitters. Simulations and experiments, including angle‐resolved FTIR and infrared thermal imaging, confirm pronounced broadband angular asymmetry in emissivity for both TE and TM polarizations. This compact, lithography compatible platform offers a scalable route to tailor thermal emission for advanced photonic systems.