Non-equilibrium Fluctuational Quantum Electrodynamics: Heat Radiation, Heat Transfer, and Force

TL;DR

This paper reviews the principles and recent advances in non-equilibrium fluctuational quantum electrodynamics, focusing on near-field heat radiation, transfer, and forces, highlighting phenomena beyond classical laws at microscopic scales.

Contribution

It introduces a scattering formalism within fluctuational QED to analyze heat radiation, transfer, and forces in non-equilibrium conditions for various materials and geometries.

Findings

Near-field heat transfer can be orders of magnitude larger than classical predictions.

Non-equilibrium Casimir forces depend on temperature differences and material properties.

Traditional laws like Stefan–Boltzmann are insufficient at microscopic scales.

Abstract

Quantum and thermal fluctuations of electromagnetic waves are the cornerstone of quantum and statistical physics, and inherent to such phenomena as thermal radiation and van der Waals forces. While the basic principles are the material of elementary texts, recent experimental and technological advances have made it necessary to come to terms with counterintuitive consequences of electromagnetic fluctuations at short scales -- in the so called {\it near-field} regime. We focus on three manifestations of such behavior: {\bf (i)} The Stefan--Boltzmann law describes thermal radiation from macroscopic bodies, but fails to account for magnitude, polarization and coherence of radiation from small objects (say compared to the skin depth). {\bf (ii)} The heat transfer between two bodies at similar close proximity is dominated by evanescent waves, and can be several orders of magnitude larger than the classical contribution due to propagating waves. {\bf (iii)} Casimir/van der Waals interactions are a dominant force between objects at sub-micron separation; the non-equilibrium analogs of this force (for objects at different temperatures) have not been sufficiently explored (at least experimentally). To explore these phenomena we introduce the tool of fluctuational quantum electrodynamics (QED) originally introduced by Rytov in the 1950s. Combined with a scattering formalism, this enables studies of heat radiation and transfer, equilibrium and non-equilibrium forces for objects of different material properties, shapes, separations and arrangements.