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

Can we overcome the fundamental blackbody limit imposed by Kirchhoff's law via passive nonlinear upconversion of thermal energy? 

We show that the fundamental limit on the blackbody radiation spectrum imposed by Kirchhoff's law (emissivity <= 1) can be surpassed through resonantly enhanced nonlinear interactions of thermal photons in a suitably engineered photonic system. 
As shown in the figure, the blackbody distribution at T=600K is centered around mid-IR wavelengths (3-10um). In a resonant system, the exponentially suppressed thermal radiation at near-IR wavelengths (1-3um) can be enhanced beyond its linear limit via nonlinear upconversion process as indicated. This has important implications in the context of renewable energy-conversion technologies. 
Opt. Exp. 2020

Photonic heat current and spin angular momentum despite uniform temperature. How is this possible? 

When the entire system is at thermal equilibrium, intuitively, there should not be any heat flow. However, in the vicinity of certain nonreciprocal materials, nonzero local heat current can exist at thermal equilibrium. It will form a closed loop, conserving energy globally and maintaining thermal equilibrium. It will not violate thermodynamic laws. 
In this work, we theoretically show that the near-field thermal radiation of certain nonreciprocal planar slabs can exhibit not only nonzero heat current but also nonzero spin angular momentum of photons without any temperature difference. The heat current is analogous to persistent electric currents that can exist without any voltage difference. We are currently working on an experiment (shown in the figure) to demonstrate it using Brownian particles.  
New J. Phys. 2019