On correlation lengths of thermal electromagnetic fields in equilibrium and out of equilibrium conditions

TL;DR

This paper investigates the spatial coherence and correlation lengths of thermal electromagnetic fields near heated surfaces, revealing differences between equilibrium and out-of-equilibrium conditions and proposing a formula for accurate correlation length calculation.

Contribution

It introduces a new formula for calculating correlation lengths of thermal fields that accounts for evanescent and propagating contributions and their mutual compensation.

Findings

Correlation lengths differ significantly between equilibrium and out-of-equilibrium states.

Evanescent and propagating contributions can mutually compensate, affecting correlation functions.

The proposed formula aligns well with observed behavior of thermal field correlations.

Abstract

Spatial coherence of thermal fields in far- and near-field zones generated by heated half-space into vacuum is studied at essentially different thermodynamical conditions. It is shown that correlation lengths of fields in any field zone are different in equilibrium and out of equilibrium systems. In wide range of distances from sample surface correlation functions are should be calculated using a total sum of evanescent and propagating contributions due to their mutual compensation at some conditions because of anticorrelations. It is demonstrated that correlation lengths as calculated with a proposed formula are in agreement with a behavior of correlation functions of thermal fields in spectral range of surface excitations