Visible to near-infrared thermal radiation from nanostructured tungsten anntennas

Abstract

We report thermal radiation antennas that sustain large emissivity at a broad range of wavelengths and angles, as well as maintaining their emissivity at high temperatures (up to 1300 K) under atmospheric pressure. Two-dimensional arrays composed of tungsten (W) cones with high aspect ratio were fabricated by laser interference lithography followed by deep reactive-ion etching, then optically characterized by measuring the broadband (λ = 500-2500 nm) absorptivity spectra. The fabricated W radiation antennas yielded greater absorptivity in visible to near-infrared wavelengths by increasing the aspect ratio of the cones, resulting in a near two-fold enhancement in average absorptivity compared to a reference planar W substrate. The measured absorptivity spectra were reproduced well by electromagnetic simulations with the experimental optical constants of W. Electromagnetic simulations also verified that such broadband increases in absorptivity are mostly caused by the enlarged scattering cross-section of individual cones. A 120 nm-thick conformal alumina coating prepared by atomic layer deposition was employed to prevent surface oxidation of the fabricated W radiation antennas, which additionally improved their absorptivity through an amplified antenna effect. These experimental and theoretical findings are generalizable to different infrared spectra by simply scaling up the antenna structures, and thus will be extensively utilized in thermal radiation applications such as solar steamers, thermophotovoltaics, and radiative coolers.