Nonreciprocal thermal metamaterials: Methods and applications

The nonreciprocity of thermal metamaterials holds significant potential applications in isolation protection, unidirectional transmission, and energy harvesting. However, due to the inherent isotropic diffusion law of heat flow, it is extremely difficult to achieve nonreciprocity of heat transfer. This review presents the recent advancements on thermal nonreciprocity. The second section explores the fundamental theories that underpin the design of nonreciprocal thermal metamaterials: Onsager reciprocity theorem. Subsequently, the third, fourth and fifth sections elucidate three methods for realizing nonreciprocal metamaterials in the thermal field: through nonlinearity, spatiotemporal modulation, and angular momentum bias, as well as the applications of nonreciprocal thermal metamaterials. In sixth section, we discuss nonreciprocal thermal radiation. In seventh section, we discuss the potential applications of nonreciprocity to other Laplacian physical fields. Finally, prospects for advancing nonreciprocal thermal metamaterials are discussed including advancements in device design and manufacturing techniques as well as machine learning-assisted material design.