Apparent spectral shift of thermally generated surface phonon-polariton resonance mediated by a non-resonant film

TL;DR

This study explains how a non-resonant film can cause an apparent spectral shift in surface phonon-polariton resonance, affecting thermal emission and flux in near-field applications.

Contribution

It reveals the physical mechanism behind the spectral shift of SPhP resonance mediated by a non-resonant film using fluctuational electrodynamics.

Findings

LDOS exhibits a local maximum near SPhP frequency due to gap modes

Spectral shifts are bounded by SiC phonon frequencies

Gap modes significantly impact near-field thermal flux and spectroscopy

Abstract

The physical origin of spectral shift of thermally generated surface phonon-polariton (SPhP) resonance of a silicon carbide (SiC) bulk mediated by a non-resonant film is elucidated. The local density of electromagnetic states (LDOS) in a non-resonant intrinsic silicon (Si) film due to thermal emission by SiC, derived using fluctuational electrodynamics, exhibits a local maximum near SPhP resonant frequency in addition to a lower frequency resonance generated by gap modes emerging in the vacuum gap separating the SiC and Si layers. Multiple reflections within the vacuum gap also induce a LDOS drop around SPhP resonant frequency. As a result, depending on the film thickness to vacuum gap ratio and the location where the LDOS is calculated in the film, the low-frequency resonance can dominate the LDOS, such that SPhP resonance appears to be redshifted. A similar spectral behavior is observed on the monochromatic radiative heat flux absorbed by the Si film. It is shown that apparent spectral (red and blue) shift of SPhP resonance mediated by a non-resonant film is bounded by the transverse and longitudinal optical phonon frequencies of SiC. This work is of importance in applications involving dissimilar materials, such as thermophotovoltaics and thermal rectification, where gap modes may significantly disrupt flux resonance. Gap modes may also be at the origin of the resonance redshift systematically observed in near-field thermal spectroscopy.