Optimal materials for maximum near-field radiative heat transfer

Abstract

We consider the space of all causal bulk materials, 2D materials, and metamaterials for maximum near-field radiative heat transfer (RHT). 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 10X 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 300K temperature, suggesting a new opportunity for plasmonics at mid- to far-infrared wavelengths, with low carrier concentrations and no requirement to minimize loss.