The study focuses on reliable reverse engineering of electron-beam deposited TiO2/SiO2 coatings. It is known that optical constants of evaporated TiO2 films are dependent on deposition conditions and may vary from layer to layer. Also, the nominal optical constants, used during the theoretical designing, may differ from the actual optical constants of coating layers, determined based on characterization of thicker single layers. Typically, post-production characterization of e-beam evaporated coatings is based on spectral photometric or/and ellipsometric data measured ex-situ. The study reports a new reliable algorithm that allows reliable estimation of layer thicknesses and optical constants based on ex-situ measurements. The reliability of the results is verified using a specially produced unique set of samples including single layers identical to the ones included in the multilayer sample. The obtained results, based on the photometric and ellipsometric data, are in correspondence with each other. The algorithm delivers practical results and avoids overfitting.
In this work is studied the influence of annealing on metal island and compact Au films formed on the substrates with and without Ge seed layer. Samples with different thicknesses of Au were annealed at different temperatures. Optical characterization of the samples was performed based on ellipsometric measurements. The obtained results demonstrate that, even in the conditions of moderately elevated temperature, Ge seed layer promotes the percolation of metal islands.
Thin metal island films exhibit unique optical properties and possess a high potential in design and fabrication of
multilayer coatings with sophisticated spectral performances over wide wavelength and angular ranges. Optical
properties of these films are dependent on film thickness. In the present study we consider and solve a problem
of designing multilayers which reflect different colors from their front and back sides and have specified average
transmittance values. Additionally, in many cases the reflected colors are stable to variations of the incidence
angle. In the design process we use optical constants of Ag metal island films, that were carefully determined
based on recently proposed characterization approach.
The influence of the substrate temperature on the effective optical behavior of Ag-SiO2 composites obtained by electron beam evaporation was studied. Optical characterization of the composites was performed by means of spectroscopic ellipsometry measurements. The effective dielectric function of the composites, modeled using a multiple oscillator approach, could be widely tuned by controlling the deposition temperature. The spectral dependence of the composite absorption appeared to be better described with a Gaussian line shape than with the classical Lorentz oscillator model. The description of the effective dielectric function using standard effective medium theories failed and the experimental results could be explained only in the general framework of the Bergman spectral density theory.
Metal island films show unique optical properties owing to the local surface plasmon resonance of islands free electrons.
In the present study, the electric field assisted dissolution of clusters in metal island films is reported. Island films of Au,
Ag, and Cu are deposited under different conditions by thermal evaporation and coated with thin dielectric layers. The
samples are treated with the simultaneous application of an intense electric field and temperature, leading to the sample
partial or total bleaching due to the dissolution of metal clusters in the films. Owing the facility of production of metal
island films and the inexpensive technical requirements of the dissolution process, this approach suggests a novel path
for the production of low-cost photonic structures.
Inhomogeneous coatings are promising for superior optical properties, e.g. broadband antireflection, in comparison to conventional HL-stack designs. Although a lot of excellent theoretical work on optical behaviour of rugates and gradient index films has been done during the last decades, there is no real breakthrough in industrial fabrication. The realization of such coatings leads to an extensive and time-consuming computer-aided control, because of complicated layer designs with continuously changing refractive index gradients. We describe the design and optical performance of an omnidirectional antireflection coating that essentially represents a hybrid coating composed from homogeneous layers and linear refractive index gradient layers.
Rugate structures, as well as gradient refractive index films in general, attract a lot of interest. The gradient index systems may provide advantages in both, optical performance and mechanical properties of the optical coatings. Rugates have shown to be especially interesting for design of notch filters. A lot of theoretical work on design of rugate filters has been done in the last decades. However, only few of the designs could be deposited, which is often caused by practical problems, e.g. preparing materials with the desired refractive index values. In this paper two different gradient refractive index designs are compared to a classical high-low stack. One gradient design is synthesized by an apodized sinusoidal structure that is approximated by homogeneous sublayers. The other one is based on an apodized sinusoidal structure as well, but it is approximated by a hybrid structure, i.e. a combination of linear gradient index ramps between the lowest and the highest refractive index applicable and homogeneous layers of high index values. The two gradient designs take into account the constraints posed by limitations of the real deposition systems. Both designs are compared to a classical high-low stack and the advantages and drawbacks of each approach are commented.
During the last few years, refractive index profiles are being studied more intensively. Several papers have been written about the use of optical methods, spectrophotometry, ellipsometry, together with reverse engineering, these have yielded interesting results. Here we study the differences of a ZrO2 film grown on an amorphous substrate and that of a pre-evaporated layer of another material, Y2O3. In the first run, two glass substrates have been coated with an Y2O3 layer. In the second run a bare glass substrate and the pre-coated Y2O3 have been coated with a ZrO2 layer. Each of the materials used had exactly the same growth conditions for all layers. The only difference was the nature of the substrates. The spectra of R and T of both samples have been used for the optical characterisation. Multisample analysis with gradual introduction of new parameters has been carried out. It was found that for the appropriate modelling of the layer, grown on the pre-coated substrate, introduction of an interface layer between the two materials was necessary. The refractive index profiles of both ZrO2 layers have been determined and discussed.
Metal-dielectric coatings are able to give high rejection in a wide wavelength range due to the reflection effect of metal layers. At the same time it is possible, according to the theory of induced transmission filters, to reach a quite high transmission peak at a single wavelength, starting from a metal layer and adding appropriate dielectric matching layers. Following this theoretical approach, narrow band-pass (10nm) filters having a peak transmission of 70% and a rejection out-band with extreme wavelengths in the ratio 2.5:1, are designed for the visible-near infrared spectrum. The low number of layers makes them interesting when a spatial variable filter has to be realized in a small dimension. Examples of their optical behavior at different wavelengths are reported.
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