Extremely directional narrowband thermal emission via diffraction-critical-point-locked quasi-BIC with tunable angular divergence

Abstract

Extreme compression of the spectrum and directionality of thermal radiation is beneficial for improving the utilization efficiency of radiative energy. However, in conventional metamaterial-based thermal radiation control schemes, although narrowband emission can be achieved, the spatial divergence angle is usually passively determined by the intrinsic properties of the resonant mode, making it difficult to design it actively and continuously as an independent variable. Therefore, achieving a designable compression of the divergence angle while maintaining a narrow bandwidth remains an unresolved challenge. Here, we propose and validate an extremely directional narrowband thermal emission strategy based on a radiative-loss jump mechanism. Specifically, the resonance wavelength of a high-quality-factor quasi-bound state in the continuum (quasi-BIC) is deliberately locked to the critical point ( λ 0 = p ) at which the −1st-order diffraction channel of a periodic structure opens. Near this singular point, the radiative loss of the system exhibits a divergent response to angular variations, thereby forcing the thermal radiation energy to be confined within an extremely narrow angular window defined by the diffraction boundary. More importantly, by tuning the detuning between the resonance wavelength and the critical point, continuous and active control of the divergence angle can be achieved. This work not only demonstrates an extremely directional narrowband thermal emitter with an actively designable divergence angle but also reveals the physical mechanism by which the spatial modes of thermal radiation can be ultimately compressed by precisely locking a high- Q quasi-BIC resonant mode to the diffraction critical point and exploiting the associated radiative-loss jump. These findings provide a theoretical foundation for shifting narrowband directional thermal emitters from passive optimization toward controllable design.