Modeling the absorption behavior of solar thermal collector coatings utilizing graded alpha-C:H/TiC layers.

Wavelength selective coatings are of common use in order to enhance the efficiency of devices heated by radiation such as solar thermal collectors. The use of suitable materials and the optimization of coating layer thicknesses are advisable ways to maximize the absorption. Further improvement is achievable by embedding particles in certain layers in order to modify material properties. We focus on optimizing the absorption behavior of a solar collector setup using copper as substrate, a layer of amorphous hydrogenated carbon with embedded titanium carbide particles (a-C:H/TiC), and an antireflection coating of amorphous silicon dioxide (aSiO(2)). For the setup utilizing homogeneous particle distribution, a relative absorption of 90.98% was found, while inhomogeneous particle embedding yielded 98.29%. These results are particularly interesting since until now, absorption of more than 95% was found only by using embedded Cr but not by using the more biocompatible Ti.

Unified analytical inversion of reflectometric and ellipsometric data of absorbing media.

We present a unified two-step analytical inversion of reflectometric and ellipsometric data of absorbing media. Instead of a direct determination of the optical constants n, kappa from reflectometric or ellipsometric measurements, we first calculate the real and the imaginary part eta, gamma of the normal component of the wave vector in the absorbing medium. New and simple analytical formulas are obtained for eta and gamma in terms of rho(s), rho(p) or tan psi, delta, respectively, where rho(s), rho(p) and tan psi, delta are the measured reflectometric and ellipsometric parameters of the absorbing medium. The optical constants are then easily determined analytically again from eta, gamma. We use the new formulas to compare the sensitivity with experimental errors due to the inversion of reflectometric and ellipsometric data.

Polychromatic reflectance and transmittance of a slab with a randomly rough boundary.

We investigated four different approximation models for describing the polychromatic reflectance and transmittance of a slab with a randomly rough boundary while taking into account the coherent and the incoherent scattering of the rough boundary. Comparisons with experiments (an etched-silicon wafer) show that approximation models that apply a two-scale roughness to the randomly rough boundary and that take into account the coherent and the incoherent scattering yield better agreement and extend the range of validity of the approximation to shorter wavelengths.

Problem of ambiguity in the determination of optical constants of thin absorbing films from spectroscopic reflectance and transmittance measurements.

We propose a new and simple procedure to overcome the ambiguity in the determination of optical constants of thin absorbing films from spectroscopic reflectance and transmittance measurements. The basis for the proposed method is an error analysis with the help of an error simulation technique and an error variation technique. We show that in practice (owing to experimental errors) it is not possible to overcome the problem of ambiguity by normal-incidence spectroscopic measurements alone. At least one oblique-incidence measurement is necessary for unambiguously determining the optical constants of the film. We discuss the consequences of experimental errors of the measured transmittance and reflectance values for the determination of the optical constants.