The sunlight has been considered a promising alternative energy source because of its abundance and sustainability. A solar thermal collector can turn the solar irradiation into the usable heat, and thus, its performance highly depends on the efficiency of absorber. An ideal absorber should trap most incoming solar radiation in the visible and near-infrared spectral region, and minimize its emitted thermal energy at long wavelengths. One of the promising solutions for satisfying the aforementioned requirements is to employ periodic structured surfaces, whose tunable radiative properties were used in thermophotovoltaic devices and chemical sensors. Two-dimensional subwavelength gratings are thus proposed for the absorber surface profile in the present study. Design objectives are a broad-band peak in the absorption spectrum and a quasi-isotropic angular lobe at the incidence of both linear polarizations. Nickel is selected for its fabrication easiness and low cost. A SiO2 film sandwiched between gratings and a substrate is considered as extra design flexibility to possibly enhance performance without much difficulty. Radiative properties and electromagnetic fields will be obtained from programs based on the rigorous coupled-wave analysis (RCWA). The optimization is then realized with the Taguchi method.

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