Broadband polarization-asymmetric directional thermal emission enabled by opposite phase-gradient metasurfaces

Abstract Thermal emission from natural materials is typically broadband and weakly structured in angle and polarization, limiting its utility for customized infrared functionalities. Here we demonstrate a metasurface platform that realizes broadband polarization-asymmetric directional thermal emission across the 8–14 μm long-wave infrared (LWIR) atmospheric window by engineering opposite reflection phase gradients for two arbitrary orthogonal polarization bases. The metasurface comprises anisotropic metallic meta-atoms integrated with a lossy multilayer film that supports low-Q gap-plasmon resonances, enabling a well-resolved polarization-dependent phase gradient throughout the LWIR band. Beyond a critical angle, the phase gradient supplies the in-plane momentum required to unidirectionally excite evanescent waves, leading to strongly polarization-dependent asymmetric absorption and thus asymmetric emission via Kirchhoff’s law. Leveraging a Particle Swarm Optimization (PSO) algorithm, we optimize supercells for both linear and circular orthogonal bases, and demonstrate that one polarization preferentially emits toward positive angles while the orthogonal polarization emits toward negative angles, achieving spatial separation of thermal emission by polarization. Angle-resolved spectral measurements and polarization-resolved thermal imaging validate the predicted broadband asymmetry and its reversal between orthogonal polarizations. Our results showcase that gradient metasurfaces provide a powerful platform for multi-degree-of-freedom control of thermal emission through evanescent-wave engineering, and open a practical route to broadband, polarization-encoded thermal emission for applications including polarization-resolved infrared imaging, dual-channel thermal communication, and adaptive thermal signature control.