Nonreciprocal Thermal Radiation in Magnetic Materials

Nonreciprocal thermal radiation describes a scenario in which a magnetised or magneto‐optical material emits thermal energy differently in one direction than it absorbs from that direction, thereby violating the traditional reciprocity embodied in Kirchhoff’s law of thermal radiation. This phenomenon arises from broken time-reversal symmetry and strong gyrotropic response in materials such as magnetic semiconductors, ferrites or topological Weyl semimetals. By coupling thermal photons to magnetisation, these systems can exhibit directional and polarisation-dependent emissivity that cannot be simply inverted by reversing the propagation path. Such asymmetric emission offers profound opportunities for controlling radiative heat flow, enhancing the efficiency of thermophotovoltaic converters, enabling passive radiative cooling with directional selectivity and generating circularly polarised thermal light for sensing applications. Advances in thin-film deposition and nanostructuring have allowed experimental demonstrations of broad-band and angle-tolerant nonreciprocal emitters at infrared wavelengths, while theoretical studies have uncovered new spin-resolved balance laws for energy and angular momentum in nonreciprocal bianisotropic media. Together, these developments point to a versatile platform for active thermal management, one-way radiative diodes and novel optomechanical forces driven by asymmetric photon momentum transfer.

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Nonreciprocal Thermal Radiation in Magnetic Materials publication trend

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Technical terms

Nonreciprocal thermal radiation: Emission and absorption of heat radiation that differ when exchange directions are reversed, enabled by broken time-reversal symmetry.

Kirchhoff’s law of thermal radiation: A principle stating that, at thermal equilibrium, a body’s emissivity equals its absorptivity for every wavelength and direction in reciprocal systems.

Gyrotropy: The property of a medium in which its permittivity or permeability tensor becomes anisotropic and complex under an applied magnetic field, leading to optical circular birefringence or dichroism.

Weyl semimetal: A topological material in which low-energy quasiparticles behave as Weyl fermions, exhibiting strong nonreciprocal magneto-optical responses in the infrared.

Bianisotropic medium: A general class of electromagnetic media where electric and magnetic fields are cross-coupled, permitting asymmetric and nonreciprocal wave propagation.

References

1. Broadband nonreciprocal thermal emissivity and absorptivity. Light: Science & Applications (2024).
2. Ultra-broadband and wide-angle nonreciprocal thermal emitter based on Weyl semimetal metamaterials. Nanophotonics (2024).
3. New spin-resolved thermal radiation laws for nonreciprocal bianisotropic media. New Journal of Physics (2020).