Near-field radiative heat transfer (NFRHT) arises between objects separated by nanoscale gaps and leads to dramatic enhancements in heat transfer rates compared to the far-field. Recent experiments have provided first insights into these enhancements, especially using silicon dioxide (SiO2) surfaces, which support surface phonon polaritons (SPhP). Yet, theoretical analysis suggests that SPhPs in SiO2 occur at frequencies far higher than optimal. Here, we first show theoretically that SPhP-mediated NFRHT, at room temperature, can be 5-fold larger than that of SiO2, for materials that support SPhPs closer to an optimal frequency of 67 meV. Next, we experimentally demonstrate that MgF2 and Al2O3 closely approach this limit. Specifically, we demonstrate that near-field thermal conductance between MgF2 plates separated by 50 nm approaches within nearly 50% of the global SPhP bound. These findings lay the foundation for exploring the limits to radiative heat transfer rates at the nanoscale.

We consider the space of all causal bulk materials, 2D materials, and metamaterials for maximum near-field radiative heat transfer (RHT) between planar structures. Causality constrains the bandwidth over which plasmonic response can occur, explaining two key traits in ideal materials: small background permittivities (minimal high-energy transitions in 2D materials) and Drude-like free-carrier response, which together optimally yield 10× enhancements beyond the theoretical state-of-the-art. We identify transparent conducting oxides, III-nitrides, and graphene as materials that should offer nearly ideal near-field RHT rates, if doped to exhibit plasmonic resonances at what we term "near-field Wien frequencies". Deep-subwavelength patterning can provide marginal further gains at the expense of extremely small feature sizes. Optimal materials have moderate loss rates and plasmonic response at 19 μm for 300 K temperature, suggesting a new opportunity for plasmonics at mid-to far-infrared wavelengths, with low carrier concentrations and no requirement to minimize loss.