Thermal emission by a subwavelength aperture

TL;DR

This paper investigates how the thermal emission of subwavelength apertures differs from larger ones, revealing that small apertures have significantly reduced emission and unique temperature scaling, with implications for materials supporting surface waves.

Contribution

It provides a theoretical analysis of thermal emission from subwavelength apertures using fluctuational electrodynamics, including a simple emissivity expression and the effect of surface waves.

Findings

Subwavelength apertures have greatly reduced thermal emission compared to classical blackbodies.

Emissivity of small apertures scales as T^8, unlike T^4 for larger apertures.

Surface waves like phonon polaritons increase emissivity of small disks.

Abstract

We calculate, by means of fluctuational electrodynamics, the thermal emission of an aperture filled by vacuum or a material at temperature T. We show that thermal emission is very different whether the aperture size is large or small compared to the thermal wavelength. Subwavelength apertures filled with vacuum (subwavelength blackbody) have their thermal emission strongly decreased compared to classical blackbodies. A simple expression of their emissivity can be calculated and their total emittance scales as T 8 instead of T 4 for large apertures. Thermal emission of disk of materials with a size comparable to the wavelength is also discussed. It is shown in particular that emissivity of such a disk is increased when the material can support surface waves such as phonon polaritons.