# Thermal spin photonics in the near-field of nonreciprocal media

## TL;DR

This paper explores how nonreciprocal materials can exhibit persistent thermal photon spin and heat currents at equilibrium, revealing new phenomena in near-field thermal radiation linked to spin-momentum locking and surface polaritons.

## Contribution

It introduces the concepts of persistent thermal photon spin and planar heat current in nonreciprocal media, supported by theoretical analysis and a proposed imaging experiment.

## Key findings

- Nonzero photon spin and heat flux occur in nonreciprocal slabs at thermal equilibrium.
- Spin-momentum locking of evanescent waves underpins these phenomena.
- Surface polaritons exhibit a spin magnetic moment in nonreciprocal photonics.

## Abstract

The interplay of spin angular momentum and thermal radiation is a frontier area of interest to nanophotonics as well as topological physics. Here, we show that a thick planar slab of a nonreciprocal material, despite being at thermal equilibrium with its environment, can exhibit nonzero photon spin angular momentum and nonzero radiative heat flux in its vicinity. We identify them as the persistent thermal photon spin (PTPS) and the persistent planar heat current (PPHC) respectively. With a practical example system, we reveal that the fundamental origin of these phenomena is connected to spin-momentum locking of thermally excited evanescent waves. We also discover spin magnetic moment of surface polaritons in nonreciprocal photonics that further clarifies these features. We then propose a novel thermal photonic imaging experiment based on Brownian motion that allows one to witness these surprising features by directly looking at them using a lab microscope. We further demonstrate the universal behavior of these near-field thermal radiation phenomena through a comprehensive analysis of gyroelectric, gyromagnetic and magneto-electric nonreciprocal materials. Together, these results expose a surprisingly little explored research area of thermal spin photonics with prospects for new avenues related to non-Hermitian topological photonics and radiative heat transport.