New spin-resolved Kirchhoff's laws for nonreciprocal media
A chiral absorber of light can emit partially circularly polarized light because its spin-resolved absorptivity is equal to spin resolved emissivity according to Kirchhoff's law. However, this law is derived and valid only for reciprocal media. No such law is known for nonreciprocal media which include magneto-optic or gyroelectric materials (Weyl semimetals, metals and semiconductors in magnetic field), gyromagnetic materials (ferromagnets and ferrites), magnetoelectric materials (multiferroics, artificial metamaterials).
In this recent work, we have discovered different spin-resolved Kirchhoff's laws which are applicable to multilayered planar configurations of these generic bianisotropic nonreciprocal material classes. We also show consistency of our derivation with a fundamental principle of detailed balance of angular momentum at thermal equilibrium. This principle was previously overlooked in the derivations of Kirchhoff's laws.
Read more: NJP 2020