Nonreciprocal Thermal Photonics for Energy Conversion and Radiative Heat Transfer

Controlling emission and absorption, and radiative heat transfer, is important for photonic energy conversion and thermal management. However, the reciprocal emission and absorption and the reciprocal radiative heat transfer in systems that satisfy the Lorentz reciprocity place fundamental constraints on a range of photonic energy-conversion technologies and thermal management. Breaking the Lorentz reciprocity points to important opportunities for realizing photonic energy conversion at the thermodynamic limit and controlling radiative heat transfer at the nanoscale. In this article, we review the development of nonreciprocal thermal photonics for energy conversion and radiative heat transfer. In Sec. II, we discuss Landsberg schemes for reaching the thermodynamic limit in a range of photonic energy-conversion technologies, including harvesting incoming radiation, photovoltaics, harvesting outgoing radiation, thermophotovoltaics, and simultaneously harvesting the sun and outer space. For nonreciprocal photonic energy conversion, it is critical to design nonreciprocal emitters and absorbers. In Secs. III-V, we discuss different approaches to achieving nonreciprocal emission and absorption, including by using magnetic response time-variant systems and optical nonlinearity, respectively. In Sec. III, we discuss achieving nonreciprocal emission and absorption through magnetic response, including by applying an external magnetic field to magneto-optical materials and by using the internal magnetization in magnetic Weyl semimetals. In Sec. IV, we discuss the use of time-variant systems through time modulation for achieving nonreciprocal emission and absorption. We highlight nonreciprocal emission and absorption in a spatiotemporally modulated antenna and photonic refrigeration based on time modulation. In Sec. V, we discuss the use of Kerr nonlinearity for breaking the Lorentz reciprocity and for achieving photonic refrigeration. In Sec. VI, we discuss radiative heat transfer in nonreciprocal materials. We talk about intriguing phenomena of many-body radiative heat transfer in systems that violate the Lorentz reciprocity, including persistent heat current at thermal equilibrium, the photon thermal Hall effect, a nonreciprocal thermal diode, thermal magnetoresistance, and thermal routing. Finally, we provide remarks on challenges and an outlook on future directions in the emerging field of nonreciprocal thermal photonics.