Thermal radiation enhancement valley between subwavelength polar membranes across the far-to-near field transition

We demonstrate that the thermal radiation between subwavelength membranes of silicon nitride exhibits a minimum enhancement over the blackbody limit, for a given separation distance. Using fluctuational electrodynamics simulations, we show that this minimum appears for membranes' separation distances corresponding to the transition between the far- and near-field regimes. The observed minimum results from the weak contributions of both the evanescent and propagating electromagnetic modes, and its values become lower for higher temperatures and thicker membranes. Outside this transition, the radiative enhancement increases and saturates for sufficiently long separation distances. Notably, the difference between the minimum and saturation values diminishes as the membranes' thickness increases. The far-field plateau of radiative enhancement is primarily governed by the coupling of surface phonon-polaritons propagating along the top and bottom surfaces of sufficiently thin membranes, which expands the effective emission cross-section area beyond the membranes' geometric one. Our findings thus show that while subwavelength polar membranes enable us to significantly enhance the far-field thermal radiation, this enhancement is limited in the far-to-near field transition. This observation highlights the necessity of optimizing both the separation distance and membrane properties to enhance the radiative heat transfer between membranes.