Nonreciprocal photonics and its application in thermal radiation

Nonreciprocity denotes the asymmetrical reaction when the sources and observation sites are exchanged.Extensive approaches have been employed to construct nonreciprocal nanophotonic devices in the optical regime.Very recently, this concept was extended to the realm of thermal radiation, emphasizing its significance in overcoming the limitations of Kirchhoff's law, which asserts that spectral directional absorptivity and emissivity are identical.This facilitates a new understanding of radiative phenomena and paves the way for innovative energy devices.In this review, we summarize the principles of nonreciprocal photonics and outline two primary methods to break Lorentz reciprocity.The extension of nonreciprocal photonics into thermal radiation is highlighted, including a range of nanophotonic structures and their potential applications in photonic energy conversion.We also discuss current challenges in nonreciprocal thermal radiation and provide the outlook for future development.