Phase Shift Compensation Enables Rainbow‐Free, Ultrawide Angular, and Pattern‐Free Nonreciprocal Thermal Emission

ABSTRACT Thermal emission from a conventional blackbody is inherently incoherent. Non‐Kirchhoff thermal emitters, which exploit nontrivial nonreciprocal properties, enable unprecedented control over both the spatial and temporal coherence of thermal emission with enhanced degrees of freedom. Although violations of Kirchhoff's law have been demonstrated, achieving strong nonreciprocity while suppressing the rainbow effect across ultrawide angular ranges remains an open challenge. This study demonstrates a pattern‐free Fabry–Pérot resonator comprising Weyl semimetal/germanium heterostructures on a silver substrate that achieves rainbow‐free nonreciprocal thermal emission with exceptional angular stability. The Weyl semimetal serves dual functions: simultaneously breaking Lorentz reciprocity and providing dynamic phase shift compensation, enabling the structure to maintain nonreciprocity exceeding 0.6 or 0.8 across an ultrawide angular range of Δ θ = 75° or 60° at a wavelength of 11.17 μm, substantially surpassing current state‐of‐the‐art narrowband nonreciprocal emitters in both angular coverage and monochromaticity (Δ λ / λ 0 < 0.54%). Active tuning of the resonant wavelength is achieved through either chemical potential modulation in the Weyl semimetal or precise cavity thickness control, without compromising the rainbow‐free or ultrawide angular response. Characterization of thermal and azimuthal dependencies reveals exceptional stability across wide temperature (Δ T = 100 K) and azimuthal angular ranges (Δ δ = 60°). These findings enable simplified, low‐cost thermal emitters with potential applications in infrared imaging, sensing, and energy harvesting.