Knowledge of the optical constants of particulate samples is critical in order to accurately model their optical behavior. For example, the dispersion and attenuation of a silicate sand are required to model scattering through a dust cloud. Most methods of measuring these quantities, however, require a polished solid sample and are therefore not suited to particulates. We present a novel method of measurement based on spectroscopic ellipsometry that can be applied to any particulate material. First, an adhesive compound is prepared and polished, and its optical constants are extracted. Then, a mixture of the adhesive and a particulate sample is prepared, and, treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are determined. We test the method’s effectiveness using pure silica powder, demonstrating that the results match literature values. The method is then applied to real sand samples. We present data for several types of sand and show that it is possible to accurately determine their optical properties and to observe the Christiansen effect in these samples.
Both ordered and random anti-reflective surface structures (ARSS) have been shown to increase the transmission of an optical surface to >99.9%. These structures are of great interest as an alternative to traditional thin film anti-reflection (AR) coatings for a variety of reasons. Unlike traditional AR coatings, they are patterned directly into the surface of an optic rather than deposited on its surface and are thus not prone to the delamination under thermal cycling that can occur with thin film coatings. Their laser-induced damage thresholds can also be considerably higher. In addition, they provide AR performance over a larger spectral and angular range. It has been previously demonstrated that random ARSSs in silica are remarkably insensitive to incident polarization, with nearly zero variation in transmittance with respect to polarization of the incident beam at fixed wavelength for angles of incidence up to at least 30°. In this work, we evaluate polarization sensitivity of ARSS as a function of wavelength for both random and ordered ARSS. We demonstrate that ordered ARSS is significantly more sensitive to polarization than random ARSS and explain the reason for this difference. In the case of ordered ARSS, we observe significant differences as a function of wavelength, with the transmittance of s- and p-polarized light diverging near the diffraction edge. We present results for both silica and spinel samples and discuss differences observed for these two sets of samples.
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