Thermal Spin Photonics In Nonequilibrium And Nonreciprocal Systems

Abstract

We show new possibilities related to the spin angular momentum of thermal radiation using fluctuational electrodynamic theory. First, we reveal that a dimer of coupled nanoantennas held at unequal temperatures (nonequilibrium) can emit circularly polarized (CP) light; a mechanism fundamentally unrelated to usual concepts of linear to circular polarization conversion or structural chirality of the light source. Furthermore, the handedness can be flipped by simply interchanging the antenna temperatures, thereby enabling reconfigurability of the polarization state lacked by most CP light sources. Second, we reveal that a nonreciprocal planar slab (e.g. semiconductor or metal in magnetic field), despite being at thermal equilibrium with its environment, can lead to nonzero photon spin angular momentum and nonzero radiative heat flux in its vicinity. We identify them as the persistent thermal photon spin and the persistent planar heat current respectively. With a practical example system, we show that the fundamental origin of these phenomena is connected to the spin-momentum locking of thermally excited evanescent waves. Our work paves the way for new practical and fundamental avenues based on the interplay of photon spin and radiative heat.