Thermal radiation and near-field energy density of thin metallic films

TL;DR

This paper analyzes the thermal radiation and near-field energy density of thin metallic films using macroscopic fluctuational electrodynamics, revealing how film thickness and distance influence radiation intensity and energy density, with implications for near-field experiments.

Contribution

It provides analytical expressions for electromagnetic fields in thin films and uncovers the finite near-field energy density due to surface plasmon polaritons, advancing understanding of near-field thermal radiation.

Findings

Radiative intensity peaks at an optimal film thickness due to multiple reflections.

Near-field energy density depends on multiple length scales and remains finite for very thin films.

Results serve as a reference for near-field experimental investigations.

Abstract

We study the properties of thermal radiation emitted by a thin dielectric slab, employing the framework of macroscopic fluctuational electrodynamics. Particular emphasis is given to the analytical construction of the required dyadic Green's functions. Based on these, general expressions are derived for both the system's Poynting vector, describing the intensity of propagating radiation, and its energy density, containing contributions from non-propagating modes which dominate the near-field regime. An extensive discussion is then given for thin metal films. It is shown that the radiative intensity is maximized for a certain film thickness, due to Fabry-Perot-like multiple reflections inside the film. The dependence of the near-field energy density on the distance from the film's surface is governed by an interplay of several length scales, and characterized by different exponents in different regimes. In particular, this energy density remains finite even for arbitrarily thin films. This unexpected feature is associated with the film's low-frequency surface plasmon polariton. Our results also serve as reference for current near-field experiments which search for deviations from the macroscopic approach.