Abstract
A chiral absorber of light can emit spin-polarized (circularly polarized) thermal radiation based on Kirchhoff’s law which equates spin-resolved emissivity with spin-resolved absorptivity for reciprocal media at thermal equilibrium. No such law is known for nonreciprocal media. In this work, we discover three spin-resolved Kirchhoff’s laws of thermal radiation applicable for both reciprocal and nonreciprocal planar media. In particular, these laws are applicable to multi-layered or composite slabs of generic bianisotropic material classes which include (uniaxial or biaxial) birefringent crystals, (gyrotropic) Weyl semimetals, magnetized semiconductors, plasmas, ferromagnets and ferrites, (magnetoelectric) topological insulators, metamaterials and multiferroic media. We also propose an experiment to verify these laws using a single system of doped indium antimonide (InSb) thin film in an external magnetic field. Furthermore, we reveal a surprising result that the planar slabs of all these material classes can emit partially circularly polarized thermal light without requiring any surface patterning, and identify planar configurations which can experience nontrivial thermal optomechanical forces and torques upon thermal emission into the external environment at lower temperature (nonequilibrium). Our work also provides a new fundamental insight of detailed balance of angular momentum (in addition to energy) of equilibrium thermal radiation, and paves the way for practical functionalities based on thermal radiation using nonreciprocal bianisotropic materials.
Highlights
At the foundation of the field of thermal radiation lies Kirchhoff’s law which relates emissivity with absorptivity
We reveal a surprising result that the planar slabs of all these material classes can emit partially circularly polarized thermal light without requiring any surface patterning, and identify planar configurations which can experience nontrivial thermal optomechanical forces and torques upon thermal emission into the external environment at lower temperature
Our work provides a new fundamental insight of detailed balance of angular momentum of equilibrium thermal radiation, and paves the way for practical functionalities based on thermal radiation using nonreciprocal bianisotropic materials
Summary
At the foundation of the field of thermal radiation lies Kirchhoff’s law which relates emissivity with absorptivity. Its mathematical form is: η(+,−)(ω, n) = α(+,−)(ω, n) where (+) denotes RCP, (−) denotes LCP, ω is the frequency and n denotes the direction This spin-resolved Kirchhoff’s law is valid only for reciprocal media. Many works have used it to design reciprocal chiral absorbers [1,2,3] which can emit partially spin-polarized thermal radiation and few works have demonstrated it in experiments [4, 5]. This conventional law is not applicable for nonreciprocal media with broken time reversal symmetry such as semiconductors in external magnetic fields. We provide the spin-resolved Kirchhoff’s laws which are applicable for nonreciprocal media
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