High-Q Resonance Engineering in Momentum Space for Highly Coherent and Rainbow-Free Thermal Emission.

Thermal emission from blackbody is typically incoherent and broadband. Achieving highly coherent thermal source while eliminating the rainbow effect has been remaining a challenging task. In our study, we utilize the isolated nature of bound states in the continuum (BICs) at the Γ point to achieve thermal emission with high temporal and spatial coherence. Under the framework of temporal coupled mode theory (TCMT), we can significantly reduce the Q-factors of modes outside the Γ point by employing far-field coupling of modes in different polarization channels within momentum space, thereby suppressing the rainbow effect. Our design, experimentally validated through ternary grating structures, demonstrates thermal emission centered at 6.5 μm with a 23 nm bandwidth, confined within a 2° angular range. This advancement holds significant implications for the miniaturization and integration of thermal radiation devices, with potential applications in infrared imaging, sensing, and energy harvesting.