Broadband enhancement of near-field thermal radiation via gradient in-plane coupled mode

The narrowband resonance characteristic of surface phonon polaritons limits the performance of many polar materials in near-field radiative heat transfer (NFRHT). To overcome this limitation, we propose a gradient in-plane coupled mode. Taking SiC as a typical narrowband phonon-polaritonic material, it is demonstrated that employing this strategy can achieve a 1.59 times enhancement in heat flux that surpasses those of single-groove and equivalent-depth metasurfaces. The near-field radiative heat flux, energy transmission coefficients, and magnetic field profiles are calculated using the rigorous coupled-wave analysis. The results reveal that the enhancement is attributed to the hybridization of multiband hyperbolic modes, which are induced by coupled cavity resonances in the grooves. Furthermore, the multiband hyperbolic modes lead to multiple resonance peaks in the spectral heat flux, which are not simply additive but result from the interference effects within the structure. Our work provides a paradigm for overcoming the limitations of single-phonon materials in NFRHT.