Material scaling and frequency-selective enhancement of near-field radiative heat transfer for lossy metals in two dimensions via inverse design

The super-Planckian features of radiative heat transfer in the near field are known to depend strongly on both material and geometric design properties. However, the relative importance and interplay of these two facets, and the degree to which they can be used to ultimately control energy flow, remains an open question. Recently derived bounds suggest that enhancements as large as |χ|4λ2/4π2Imχ2d2 are possible between extended structures (compared to blackbody), but geometries reaching this bound, or designs revealing the predicted material (χ) scaling, are lacking. Here, exploiting inverse techniques, in combination with fast computational approaches enabled by the low-rank properties of elliptic operators for disjoint bodies, we investigate this relation between material and geometry on a wide variety of periodic gratings. Crucially, we find that the material proportionality given above does indeed emerge in realistic structures, at least within the range of explored values of χ. In reaching this result, we also show that (in two dimensions) lossy metals such as tungsten, typically considered to be poor candidate materials for strongly enhancing heat transfer in the near infrared, can be structured to selectively realize flux rates that come within 50% of those exhibited by an ideal pair of resonant lossless metals for separations as small as 2% of a tunable design wavelength.