Coexistence of multiple regimes for near-field thermal radiation between two layers supporting surface phonon polaritons in the infrared

Abstract

We demonstrate the coexistence of different near-field thermal radiation regimes between two layers supporting surface phonon polaritons (SPhPs) in the infrared. These regimes exist when the distance of separation between the media $d$ is much smaller than the dominant emission wavelength. This coexistence is noticed after computations of the near-field radiative heat transfer coefficient ${h}_{r}$ for silicon carbide films using fluctuational electrodynamics and following an asymptotic analysis of ${h}_{r}$. We show that the emergence of these regimes is a function of a dimensionless variable $D$ defined as the ratio of the layer thickness $t$ to $d$. When $D$ $\ensuremath{\gg}$ 1 for both films, SPhPs dominating near-field radiant energy exchange do not couple within the layers, such that ${h}_{r}$ follows a ${d}^{\ensuremath{-}2}$ power law as for the case of two planar half-spaces. When $D$ $\ensuremath{\ll}$ 1 for both layers, the dominant SPhPs couple within the films, thus resulting in a splitting of the spectral distribution of flux into two distinct modes. Despite this splitting, the asymptotic expansion reveals that ${h}_{r}$ varies as ${d}^{\ensuremath{-}2}$ due to the fact that the spectral bands of high emission and absorption are essentially the same for both films. However, when both layers have a thickness of the order of a nanometer or less, a purely theoretical regime emerges where ${h}_{r}$ follows a ${d}^{\ensuremath{-}4}$ asymptote. Also, when one layer has $D$ $\ensuremath{\ll}$ 1 while the other one is characterized by $D$ $\ensuremath{\gg}$ 1, there is an important mismatch between the spectral bands of high emission and absorption of the films, thus resulting in a ${h}_{r}$ varying as ${d}^{\ensuremath{-}3}$. These various near-field thermal radiation regimes are finally summarized in a comprehensive regime map. This map provides a clear understanding of near-field thermal radiation regimes between two layers, which are particularly important for designing highly efficient nanoscale-gap thermophotovoltaic power generation devices.