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

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 >> 1 for both films, SPhPs dominating near-field radiant energy exchange do not couple within the layers, such that h(r) 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 h(r) 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 h(r) 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.