Enhanced near-field thermal radiation in multilayered hBN-graphene structures without vacuum gaps

Near-field thermal radiation enables heat transport that greatly exceeds the blackbody limit through photon tunneling of evanescent electromagnetic waves at nanoscale separations, and has attracted increasing attention as an alternative heat-dissipation mechanism for miniaturized devices. In this study, we focus on solid-spacer multilayer structures without vacuum gaps, and numerically compare a hyperbolic material, hexagonal boron nitride (hBN), with an isotropic polar material, silicon carbide (SiC), in structures composed of alternating graphene sheets and dielectric spacers. Numerical analysis shows that, at N = 5, the hBN-graphene multilayer structure achieves a total heat flux of 656 kW/m2, approximately 3.0 times that of the SiC-graphene structure (221 kW/m2) under the same conditions, and approximately 6.5 times that of the corresponding vacuum-gap hBN-graphene structure. Analysis of the photon tunneling probability maps reveals that this enhancement originates from hybrid modes formed by coupling between hyperbolic phonon polaritons excited within the Reststrahlen bands of hBN and graphene surface plasmon polaritons. We explicitly identify these hybrid branches in the present vacuum-gap-free, solid-spacer configuration, enabling efficient energy transport through high-wavevector near-field radiative channels. We further confirm that branch splitting of the polaritonic modes associated with multilayering increases the effective density of transmission channels. These results demonstrate that solid-spacer multilayer hBN/graphene structures constitute an effective design strategy for enhancing near-field thermal radiation without requiring vacuum gaps.