Suppression of Radiative Heat Transfer Between Two Metasurfaces

Heat can transfer between two objects through conduction, convection, and radiation. In many applications, efficient heat transfer is desirable, such as in the cooling systems for electronics, to ensure reliable operations. Conversely, it is imperative to suppress heat transfer in certain applications. While thermal conduction and convection can be easily controlled, the ubiquity of thermal radiation from materials at finite temperature poses challenges to engineering solutions aimed at suppressing radiative transfer. For instance, for a superconducting material working at ~2 K, its intrinsic radiation is minimal and any radiative heat transfer would have a significant impact, which is undesirable. For the suppression of radiative heat transfer, the most common approach is to coat a metal layer to reduce emissivity, thereby reducing heat exchange. Yet, adding another metal layer may interfere with the designed electronic functionalities. Alternatively, distributed Bragg reflectors (DBRs) and other photonic structures using dielectric materials have been employed to reduce emissivity compared to bulk systems. However, such designs are mostly limited to a narrow frequency range, as any photonic resonance that restrains emission at one frequency inevitably enables the emission in other frequencies. Therefore, it is beneficial to develop a system that could alleviate these bandwidth restrictions and effectively suppress the radiative heat transfer across a wide frequency range. However, there are fundamental constraints on the bandwidth of passive DBRs which prevent such functionality.<br/>Here, we demonstrate optimal DBR design pairs that suppress heat transfer between two finite-sized planes over broad bandwidths. Rather than aiming at maximizing the continuous bandwidth of high reflectivity for each DBR, we achieve this feat by mis-aligning the peaks and dips of the DBR pair, making it mutually incompatible for heat transfer. Notably, we co-optimize the two-DBR response using an end-to-end inverse design algorithm. We experimentally validate the effectiveness of the design by measuring angular-resolved emissivity and calculating the radiative heat transfer between the two slabs. Moreover, we extend the theory to near-field heat transfer in this scenario, and investigate the fundamental limit of this approach. In summary, our co-optimized metasurfaces effectively suppress radiative heat transfer over broad bandwidths, demonstrating their potential in thermal management across various applications, including cryogenic radiative insulation, energy harvesting, and cooling technologies.