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 existwhen 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 hr for silicon carbide films using fluctuational electrodynamics and following an asymptotic analysis of hr . 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 _ 1 for both films, SPhPs dominating near-field radiant energy exchange do not couple within the layers, such that hr follows a d−2 power law as for the case of two planar half-spaces.When D_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 hr varies as d−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 hr follows a d−4 asymptote. Also, when one layer has D _ 1 while the other one is characterized by D _ 1, there is an important mismatch between the spectral bands of high emission and absorption of the films, thus resulting in a hr varying as d−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.