Angle-asymmetric thermal radiation via efficient inverse design of multi-layer magneto-optical films

The broad-band, non-directional, and reciprocal properties of traditional thermal radiation significantly limit the precise control and efficient utilization of thermal radiation energy. In recent years, nonreciprocal thermal radiation (NTR) has been proposed with a magneto-optical effect or spatiotemporal modulation; however, most existing NTRs are only achieved with a specific zenith angle (θ) without considering the azimuthal angles (φ), which actually is a nominal NTR and not enough for practical angularly asymmetric thermal radiation (ATR) control. Therefore, we employ the Monte Carlo tree algorithm to optimize the multi-layer structure with magneto-optical materials, which can realize ATR with narrow azimuth (φ: 165°–195°) at a given zenith angle (θ = 30°) and wavelength (λ = 16 μm). The underlying mechanism is primarily attributed to the excitation of the magneto-optical effect and cavity resonance, which is corroborated by the analysis of a magnetic field distribution. Furthermore, we investigate the impact of the layer number and the zenith angle on the ATR. The proposed design algorithm is, in general, that can be extended to any given zenith angle and azimuthal angle and also paves the way for novel applications in areas, such as directional radiative cooling and thermal focusing.