Near-field thermal radiation between deep subwavelength membranes driven by corner and edge modes

We demonstrate that the thermal radiation between deep subwavelength membranes of silicon carbide (SiC) exhibits a maximum enhancement over that of infinite SiC surfaces separated by the same vacuum gap. Based on fluctuational electrodynamics, we show that this enhancement occurs at a separation distance of 200nm and increases for thinner and colder membranes. This peak arises from the dominant contribution of electromagnetic modes localized at the corner and vertical edges of sufficiently thin membranes, which enable a strong coupling of surface phonon-polaritons appearing along their top and bottom surfaces. These resonant corner and edge modes effectively extend the emission cross-sectional area of the membranes over their geometrical one and, therefore, amplify their thermal radiation. For 10-nm-thick membranes of SiC at 300 K, the thermal conductance reaches 54pWK−1, which yields a maximum enhancement of 4.5 over the value for infinite SiC surfaces. Our findings, thus, reveal that the regime of near-field thermal radiation driven by corner and edge modes emerges and is optimized in deep subwavelength membranes separated by intermediate distances.