We study different effective medium theories for describing the optical behaviour of composites consisting of spherical metallic inclusions embedded in a dielectric matrix. The analysis is performed according to the Bergman spectral density theory. This theory establishes that any effective medium model has an integral representation in terms of a function (the spectral density) that depends on the geometry of the two-phase mixture and is independent of the optical constants of the composing materials. We review classical effective medium theories (Maxwell-Garnett and Bruggeman models) according to their spectral density. Furthermore, numerical simulations based in recent works allow studying the influence of different geometric parameters in the spectral density and compare the results with the classical theories.
During the recent years, there has been a growing interest on the physical properties of zirconium dioxide (ZrO2) for its possible use as high-k material and its application in optical coatings technology as high-refractive index material. In the present work we study the optical and structural properties of ZrO2 thin films obtained by plasma ion-assisted deposition (PIAD) on silicon wafers, in their as-deposited state and after annealing of the samples at different temperatures. The optical properties were studied by variable angle spectroscopic ellipsometry in the visible spectral range, while the structural properties were analyzed with grazing-incidence x-ray diffraction and x-ray reflectometry. The experimental results show a clear correlation between the optical properties and the variations of the structural properties due to the annealing. Thus, the as-deposited layers show a poor crystalline state, with a low refractive index and energy band-gap. As the annealing temperature was augmented, the degree of crystallinity was increased, as well as the refractive index and the band-gap. Moreover, the annealing also induced a reduction of the layer thickness and a slight increase of the surface roughness.
The time evolution of the optical properties of nanostructured silicon nitride (ns-SiNx:H) thin films was studied by FTIR
phase-modulated ellipsometry. The samples were produced by RF-PECVD and ellipsometric measurements were performed after the deposition and at different time intervals in the spectral range between 950 and 3500 cm-1. The experimental data show an evolution from an initial not-oxidized state to a final oxidized state. The oxidation process of ns-SiNx:H films is modeled with two different approaches: i) assuming that the oxidation starts at the film surface and diffuses towards the substrate and ii) assuming a homogeneous oxidization through the entire volume of the film. The final best fitting of the data suggests that the oxidization occurs homogeneously in all the thickness of the film.
The optical behavior of multilayer coatings for the VUV region is determined by dispersion and absorption of the used materials, non-ideal interfaces between layers and thickness of each component layer. Therefore, an accurate characterization of a multilayer stack encounters the inherent difficulties related to dealing with a high number of defining parameters. The use of on-line data (data acquired during the deposition process) allows a more precise characterization of the coatings, in comparison with the use of standard spectrophotometric measurements obtained after the deposition of the layers. Here we present the results of the characterization of a high reflecting coating where the component materials are fluorides (LaF3 and MgF2). Several models with an increasing degree of complexity and assuming different multilayer growing concepts have been applied for the description of the analyzed sample.
This paper is a work-in progress report on the development of sol-gel coatings for high power laser systems in the near-UV, infrared region. Silica, titania and titania-silica acid catalysed sols were prepared by using tetraethoxysilane and titanium isoproxide as precursors. Single and multi-layer coatings were generated by dipping on fused silica substrates. The single films were heated at 500°C and 900°C after deposition in order to investigate the role of the sintering temperature either on the optical properties and on the film laser-induced damage threshold at 1064 nm (Nd:YAG c.w. laser) and 351 nm (XeF excimer laser). The ageing effects due to the exposure to humidity was investigated by testing the damp heat resistance of the coatings in agreement with the ISO environmental test for optical coatings. The silica coatings have been assessed before and after the damp heat test with regard to their laser-induced-damage resistance, reflectance and transmittance properties. The optical parameters (refractive index and extinction coefficient) have been determined by UV-VIS-NIR spectrometry. A global fit procedure based on the simultaneous characterisation of several samples was used for the evaluation of the optical properties of the materials both as single films and inside multi-layer stacks.
Optical properties of materials inside a multilayer stack may differ from those of single layers. This fact is specially noticeable in the case of sol-gel films obtained by dip coatings. We propose a method to determine the refractive index and thickness of materials inside a multilayer stack, based on the simultaneous characterization of several samples. The procedure increases the reliability of the results and helps in improving quality in optical coating production. (Summary only available)