New spin-resolved thermal radiation laws for nonreciprocal bianisotropic media

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.

Broadband circularly polarized thermal radiation from magnetic Weyl semimetals

“…It is divided by 2 in Eq.6 to account for two orthogonal polarization states separately. Since we are interested in the circular polarization of light, the polarization-dependent emissivity is calculated in the eigenbasis of right circularly polarized (RCP) and left circularly polarized (LCP) photons [28]. Based on the definitions of the polarization eigenstates in vacuum and the definition of the spin angular momentum density [59][60][61], a single RCP photon carries − angular momentum and a single LCP photon carries + angular momentum along the propagation direction (see appendix for convention details).…”

“…The spin-resolved emissivities η (LCP/RCP) in the above expressions for nonreciprocal planar media have been derived previously in Ref. [28], and are separately discussed in the appendix (Section IV) in the context of this work. In Fig.…”

“…Although linearly polarized thermal radiation has been demonstrated using patterned or microstructured geometries [19,20], realizing and detecting circular polarization of thermal emission is more difficult and has been demonstrated in very few experiments using either a chiral absorber metasurface [21] or a polarization conversion metasurface [10]. Apart from these experiments and many theoretical proposals relying on nanophotonic design of reciprocal chiral absorbers [22][23][24], new approaches based on nonequilibrium coupled antennas [25] and nonreciprocal media [26][27][28] have been recently explored for CP thermal emission. However, in all of these works so far, either the purity of circular polarization is low or high-purity circular polarization is achieved over configuration-dependent narrow spectral and angular bandwidths.…”

“…It does not hold true for nonreciprocal media such as MWS considered here. Interestingly, nonreciprocal media follow different modified forms of spin-resolved Kirchhoff's laws [28]. The theoretical techniques and computational tools for analyzing the spin or circular polarization of thermal radiation from nonreciprocal bodies are only recently developed in the field of thermal radiation [28,54,55].…”

“…Interestingly, nonreciprocal media follow different modified forms of spin-resolved Kirchhoff's laws [28]. The theoretical techniques and computational tools for analyzing the spin or circular polarization of thermal radiation from nonreciprocal bodies are only recently developed in the field of thermal radiation [28,54,55]. All prior inquiries concerning the topic of thermal spin photonics analyzed reciprocal media for which analytic derivations and computational simulations are simplified by invoking the principle of electromagnetic reciprocity.…”