Material Scaling and Frequency-Selective Enhancement of Near-Field Radiative Heat Transfer for Lossy Metals in Two Dimensions via Inverse Design

TL;DR

This paper demonstrates how inverse design techniques can optimize lossy metallic structures in two dimensions to significantly enhance near-field radiative heat transfer, approaching theoretical bounds and revealing material and geometric interplay.

Contribution

It introduces an inverse design approach combined with fast computational methods to explore and realize material and geometric effects on near-field heat transfer in lossy metals.

Findings

Material proportionality scaling is confirmed in realistic structures.

Lossy tungsten structures can achieve near-field flux rates close to ideal lossless metals.

Enhanced heat transfer approaches theoretical bounds within 50% for small separations.

Abstract

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 $|\chi|^4 \lambda^{2} / \left(\left(4\pi\right)^{2} \Im\left[\chi\right]^{2} d^{2}\right)$ are possible between extended structures (compared to blackbody); but neither geometries reaching this bound, nor designs revealing the predicted material ($\chi$) scaling, have been previously reported. 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 an enlarged space structures. Crucially, we find that the material proportionality given above does indeed emerge in realistic structures. 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.