Thermal equilibrium spin torque: Near-field radiative angular momentum transfer in magneto-optical media

TL;DR

This paper demonstrates that thermal radiation from nonreciprocal magneto-optical bodies can exert spin torques even at thermal equilibrium, revealing new mechanisms for angular momentum transfer at the nanoscale.

Contribution

It introduces a theoretical framework combining fluctuational electrodynamics and optical angular momentum to analyze near-field radiative angular momentum transfer in magneto-optical media, including equilibrium conditions.

Findings

Single magneto-optical particle experiences torque with magnetic field.

Two particles with misaligned axes experience equal and opposite torques.

Results applicable to semiconductors like InSb and Weyl semi-metals.

Abstract

Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-sized bodies comprising of nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-sized nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in non-equilibrium with its surroundings experiences a torque in the presence of an applied magnetic field (T-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropic axes experience equal magnitude torques with opposite signs which tend to align their gyrotropic axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magneto-plasmas) as well as Weyl semi-metals which exhibit the anomalous Hall effect (gyrotropy) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.