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Optical modeling of nanostructured films for selective coatings

To improve the efficiency of solar thermal collectors, selective coatings need to be maximally transparent to sunlight (up to 2500 nm wavelength) and maximally reflective to heat radiation. One way to improve the transparency for sunlight without compromising the reflective properties at longer wavelengths is structuring thin metal films with holes. The purpose of this paper is to develop computer model capabilities to predict the optical properties of such structures by solving the Maxwell and materials equations using the finite element method in three dimensions. Coupling both sets of equations enables us to incorporate the full dispersion of metals, including their negative real part of permittivity. The finite element model is validated in two ways: firstly, by simulations of structured films using the FDTD method in the range of positive permittivity; and secondly, by the transmission and the ellipsometric measurement of homogeneous films. The simulations predict that holes with a diameter between 300 and 500 nm - and aligned in arrays with a mutual distance between 500 and 800 nm - significantly improve the performance of selective layers used for solar thermal collectors.