Multi-band and wide-angle nonreciprocal thermal radiation

Violating Kirchhoff's radiation law through magneto-optical materials or spatiotemporal (Floquet) meta-materials can open a new door for engineering thermal radiation by breaking the widely-accepted equal constraint of spectral absorptivity (alpha) and emissivity (epsilon). Most existing work only reports the unequal alpha and epsilon spectra in one or two bands and within limited angles. This significantly limits the practical applications like the nonreciprocal thermophotovoltaics. In this work, we present a general machine-learning-kernel-based algorithm framework, based on which we achieve four-band nonreciprocal thermal radiation via the magneto-optical (MO) materials. The realization of multi-band nonreciprocity is mainly attributed to the coupling effect of magneto-optical effect and the excitation of cavity modes with different orders, which can be confirmed by investigating the magnetic field distribution. In addition, it is found that dual-band/multi-band strong nonreciprocal thermal radiation can be realized in a wide range of incident angles (15 degrees-85 degrees). The number of bands and range of angle can be further enhanced by modulating the number of layers, structures, materials, and applied magnetic field. The present work offers a general design roadmap for nonreciprocal thermal radiation, and can be extended for designing metamaterials beyond thermal metamaterials. (c) 2023 Elsevier Ltd. All rights reserved.