Near-field thermal radiation of metamaterials/metasurfaces constructed of natural anisotropic materials

As one of the three fundamental heat transfer modes, thermal radiation is one of the most universal phenomena in nature. With respect to the characteristic wavelengths involved in thermal radiation, it may differentiate between the so-called far-field and near-field thermal radiation. Near-field thermal radiation enables surpassing blackbody limit and shows promising applications in a variety of fields from electronic devices to renewable energy utilization. Extensive investigations have been focused on isotropic materials, while anisotropies can provide additional routes to modulate and enhance near-field thermal radiation. This chapter introduces very recent investigations on near-field thermal radiation of metamaterials and metasurfaces made of natural anisotropic materials, including anisotropic films, metamaterials, two-dimensional materials, and corresponding nanostructured metasurfaces. Effects of axis orientations, film thickness, patterning directions, and shapes of nanostructures on tuning near-field radiative heat flux of anisotropic materials are briefly discussed. How anisotropies change the excitation condition of surface resonances and affect tunneling of evanescent waves are elaborated. Advantages of anisotropies and nanopatterns on enhancing and modulating near-field radiative heat flux are also discussed. Nevertheless, more efforts are still needed to conduct experimental demonstrations and apply them in practical energy harvesting and conversion devices.