Control Bandwidth and Contrast Between Emissivity and Absorptivity for Broadband Nonreciprocal Emitters

Broadband nonreciprocal emitters are indispensable in next-generation thermal and photonic applications, such as thermal camouflage and energy harvesting. Recently, multilayer structures consisting of magneto-optical materials with gradient doping levels have been explored for broadband nonreciprocity provided by a broadband ENZ effect. However, previous works all focus on achieving a monotonic contrast between emissivity and absorptivity, i.e., either the absorptivity or the emissivity is always larger than the other for a given direction over a broad frequency range. Here, we report that the contrast between emissivity and absorptivity of such multilayer nonreciprocal thermal emitters can be highly controllable by coupling a Fabry-Perot mode associated with an additional dielectric layer, making the contrast alternating positive and negative across the broad nonreciprocal bandwidth. Thus, the monotonic nonreciprocal bandwidth can be divided into multiple separated bands with alternating contrasts, and the number of the separated bands is proportional to the thickness of the dielectric layer. In addition, the introduction of the dielectric layer also can increase the absolute contrast between absorptivity and emissivity. Our approach of coupling a Fabry-Perot mode with ENZ effect represents a simple yet effective approach to obtaining multiband nonreciprocal thermal emitters, providing interesting new opportunities in thermal applications such as thermal circulators and isolators.