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Nanoantennas and their arrays (metasurfaces) provide a versatile platform for controlling the coherence of thermal emission. Conventional designs rely on global heating, which impedes emission efficiency and on-chip integration. In this work, we propose an electrically driven metasurface composed of a Yagi-Uda nanoantenna array interconnected by S-shaped electrode wires, which enables the concurrent manipulation of thermal emission spectrally and directionally. A direct simulation approach based on the Wiener-chaos expansion method is employed for quantitative analysis. Our metasurface device exhibits a narrowband emission with high directivity, which is one order higher than that of a single nanorod antenna case. The modeling framework established in this work opens a promising route for realizing coherent mid-infrared emission by metasurfaces. 

Abstract Orthorhombic molybdenum trioxide (α‐MoO₃) is a highly anisotropic hyperbolic material in nature. Within its wide Reststrahlen bands, α‐MoO₃has hyperboloidal dispersion that supports bulk propagation of high‐k phonon polariton modes. These modes can serve as energy transport channels to greatly enhance radiative heat transfer inside the material. In this work, large radiative transfer enabled by phonon polaritons in α‐MoO₃is demonstrated. The study first determines the temperature‐dependent permittivity of α‐MoO₃from polarized Fourier‐Transform Infrared (FTIR) spectroscopy measurements and then uses a many‐body radiative heat transfer model to predict the equivalent radiative thermal conductivity of hyperbolic phonon polariton. Contribution of radiative transfer to the total thermal transport is experimentally determined from the Time‐Domain Thermoreflectance (TDTR) measurements in a temperature range from −100 to 300 °C. It is found that radiative transfer can account for ≈60% of the total thermal transport at a temperature of 300 °C. That is, conductive thermal transport is enhanced by >100% by radiative transfer, or radiation inside α‐MoO₃is greater than that of conduction. These additional energy pathways will have important implications in thermal management in new materials and devices. 

Abstract Radiation as a heat transfer mode inside a bulk material is usually negligible in comparison to conduction. Here, the contribution of radiation to energy transport inside a hyperbolic material, hexagonal boron nitride (hBN), is investigated. With hyperbolic dispersion, i.e., opposite signs of dielectric components along principal directions, phonon polaritons contribute significantly to energy transport due to a much greater number of propagating modes compared to that in a normal material. A many‐body model is developed to account for radiative heat transfer in a material with a nonuniform temperature distribution. The total radiative heat transfer through hBN is found to be largely contributed by the high‐κ modes within the Reststrahlen bands, and is comparable to phonon conduction. Experimental measurements of temperature‐dependent thermal transport also show that radiative contribution to thermal transport is of the same order as that from phonons. Therefore, this work shows, for the first time, radiative heat transfer inside a material can be comparable to phonon conductive heat transfer.