Non-reciprocal thermal photonics—from fundamentals to applications: a tutorial

Thermal photonics has attracted considerable interest due to its ability to artificially manipulate thermal radiation. However, many thermal photonic devices are constrained by Kirchhoff’s law of thermal radiation, with identical directional emissivity and absorptivity. The rise of non-reciprocal thermal photonics offers the potential to overcome this constraint, enabling independent control of emission and absorption. This tutorial summarizes recent progress in non-reciprocal thermal photonics and the generalized Kirchhoff’s law of thermal radiation, establishing a comprehensive relationship between directional absorptivity and emissivity, irrespective of its reciprocity. It also highlights challenges in non-reciprocal thermal radiation experiments and discusses potential solutions. Providing a thorough overview of the fundamentals, experimental approaches, and applications of non-reciprocal thermal photonics, this tutorial aims to enhance understanding of its principles and facilitate advancements in energy harvesting and heat flux control.