Multi-band and wide-angle nonreciprocal thermal radiation

Violating Kirchhoff's radiation law through magneto-optical materials or spatiotemporal (Floquet) metamaterials can open a new door for engineering thermal radiation by breaking the widely-accepted equal constraint of spectral absorptivity (α) and emissivity (ϵ). Most existing work only reports the unequal α and ϵ 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°-85°). 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.