In the framework of the European research project EUFELE, a set of fluoride and oxide single layer coatings was deposited, irradiated with synchrotron radiation, and subsequently thoroughly characterised. The observed coating damage is strongly related to the spatial distribution of the synchrotron radiation. Therefore, characterisation methods have to be adapted to techniques that are capable to reveal the structural and optical behavior with adequate spatial resolution. A summary of the radiation damages of oxide materials (SiO2, Al2O3 and HfO2) produced by conventional and sputter deposition techniques, and of fluoride single layers (MgF2, LaF3, AlF3) deposited by thermal evaporation is presented. Degradation was observed within the irradiated areas as well as in the not directly exposed area. The observed degradation effects depend on the surface site. Oxide systems show a superior resistance compared to fluoride coatings. The most sensitive material is Lanthanum fluoride.
The improvement in production process of high quality optics needs routinary accurate characterization of the optical properties of the sample. Whereas some commercial instruments allow accurate measurements of the transmitted and reflected light, the measurement of the scattered light very often falls below the detection level when standard attachments, like the Ulbricht integrating sphere, are used. In order to characterize scattering levels below 10-3 dedicated set ups are usually used which are quite complicate to run and often do not fit the need of a production department. In this work we present the design and test of scatterometers based on the Coblentz sphere that fit inside a Perkin-Elmer λ-900 spectrophotometer. They are aimed to the measurement of light scattering in both the transmittion and the reflection hemisphere with a detection limit in the range of 50 ppm. The Coblentz sphere is a mirror like collector of the scattered light and does not suffer of the typical limitations of the Ulbricht sphere in terms of low collecting efficiency and spectral limitation in the UV edge. As an example, LaF3 films on silica samples were first characterized with both conventional photometric techniques (specular reflectance and direct transmittance) and ellipsometry and the film thicknesses and refractive indices, as well as inhomogeneity and interface layers, were determined. The scattering data were also independently analysed by considering the vector theory of scattering and the effect on the simulated spectral TS of several power spectrum densities obtained with different hypothesis on the correlation between interfaces.
Radiation resistance of optical materials against synchrotron radiation is important, if optical components for the high energetic regime have to be produced. In the framework of the European project EUFELE, which deals with the development of optical coatings for the free electron laser at ELETTRA (Trieste), a set of CaF2 substrates was irradiated with synchrotron radiation. The synchrotron radiation was varied by wavelength, dose, and high energetic background illumination. Before and after irradiation, the CaF2 substrates were investigated spectrophotometrically in the VUV, VIS and IR range. The surface topology was characterized by Nomarski microscope methods. Structure investigations were carried out with X-ray diffraction measurements. CaF2 shows different types of degradation like color center formation, surface modification, and increased VUV absorption bands. Defect formation will be presented in dependence of synchrotron irradiation conditions.
We present an ellipsometric measurement set up designed for fast surface mapping. The system consists of a HeNE laser, a linear polarizer, a recently developed grating polarimeter and a translation stage for moving the sample. The system allows the spatially resolved measurement of the ellipsometric angles (Delta) and (Psi) . For transparent single layer coating, refractive index and film thickness from the measured values can be calculated. For multilayer coatings where the determination of all the optical properties is not possible, information on the coating homogeneity can be gained.
The photoacoustic beam deflection technique at (lambda) equals 248 nm was employed for the measurement of the laser damage threshold of single films of magnesium fluoride deposited on superpolished calcium fluoride and fused silica substrates. Different samples were investigated. All films have similar thickness but are deposited by various techniques in different laboratories. The samples are fully characterized, both from the optical and the structural point of view, in the framework of a European Project on 'UV-coatings'. The results of these measurements are reported, along with the data on laser damage threshold, to find out which are the most significant parameters for laser applications.
Cathode sputtering is a well-known technique for deposition of thin films starting from metallic or ceramic materials. Ceramic insulating materials are usually sputtered using radio frequency as electric power source. In this work we compare the optical properties of tin films deposited from two different types of targets. The first set of samples was deposited from solid targets glued to a water-cooled back- plate. For the second set we used targets made of cold pressed powders of the corresponding materials. The possibility of sputtering from weakly pressed powders can be favorably exploited to develop mixed materials. Their cost, in fact, is about one tenth of glued solid targets, and mixtures of any required composition can be quickly prepared. Two oxides, SiO2 and Al2O3 have been deposited by both methods in the same deposition chamber, using the same process parameters, and sputtering gas. Their optical characteristics - refractive index and extinction coefficient - have been compared over the VIS-NIR spectral range.
The aim of this work is to check the possibility of using originally developed thin films of mixed oxides of cerium and vanadium as ion storage electrode within an electrochromic cell. Transmittance variation as function of the amount of lithium exchanged between the WO3 film and the Ce-V oxide film has been measured in the visible and near IR spectrum. Also real time single wavelength measurements have been performed in different conditions to test the speed of coloration.
In high quality otpical coating systems for the DUV-spectral range, MgF2 is one of the preferred deposition materials. MgF2-coatings exhibit relatively low optical losses as well as high stability and laser induced damage thresholds. In the present joint research effort of several European laboratories, the potentiality of MgF2 is evaluated in respect to the production of improved optical coatings for applications in laser technology and semiconductor lithography. For this purpose, single layers of MgF2 were deposited on superpolished fused silica and CaF2-substrates by ion beam sputtering, boat and e-beam evaporation in different laboratories. Besides photometric inspections, the samples were characterized by an optical scatter measurement facility at 193 nm and 633 nm. The structural properties were assessed using AFM, XRD, and adapted TEM-techniques invovling conventional thinning methods for the layers. For the measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed.
Recently a new approximation for the ellipsometer angles of a slightly inhomogeneous thin film was developed. This approximation enables performing a more detailed qualitative analysis of ellipsometric angles than the widely used approximation assuming the linear bulk inhomogeneity. In particular, the new approximation describes not only the impact of inhomogeneity on the ellipsometric angle (Psi) but also its impact on the ellipsometer angle (Delta) . This paper reports specific properties of ellipsometric spectra connected with the surface micro-roughness. In the particular case of fluoride films the ellipsometric angle (Delta) turns to be quite sensitive to this type of inhomogeneity if ellipsometric measurements are performed at high angles of incidence.
Films of Ce-V mixed oxide were deposited by reactive r.f. sputtering from a target of cold pressed CeO2 and V2O5 mixed powders. Optical and ion storage properties of the films have been studied in function of the oxygen partial pressure inside the sputtering chamber during the deposition process. Li intercalation was accomplished electrochemically. Optical constants have been determined for as-deposited and intercalated films.
Ellipsometers based on four-detector photopolarimeter (FDP), where the polarization of the reflected beam is characterized in terms of the Stokes vector, have been recently developed and employed as in situ diagnostic of thin film growth. Two main problems are still open. The first is the accuracy of alignment of the FDP with respect to the incident beam, the other concerns the extension of FDP to spectral applications. The base of the set-up proposed in this work is a grating photopolarimeter where a diffraction grating is use data normal incidence and two couples of detectors are arranged in the direction of the diffracted beams corresponding to the +1 and -1 orders. The working spectral range can be chosen by selecting a proper grating. The zero order is used for the autocollimation allowing accurate alignment. For this preliminary study we used a 2 mW HeNe laser as light source. The results of analytical study of the device, its computer simulation, and the examples of application to the ellipsometric measurements of thin films and bare surfaces are presented.
Cerium titanium oxide samples produced by sol-gel have been compared against sputtered and pulsed laser deposited films over a wide range of different compositions. X-ray diffraction was used to investigate the structural properties of the compound material existing in a two-phase mixture MAO2-MBO2. The optical properties were evaluated over the whole solar spectrum by variable angle spectroscopic ellipsometry combined with spectrophotometry. The spectral complex refractive index was determined for CeO2 and TiO2, as well as for their compounds. To reduce the large number of permutations in composition of multi-component oxides it would be useful to be able to predict the properties of the mixtures from the pure oxide components. Therefore these results were compared to those obtained by effective medium theory utilizing the optical constants of CeO2 and TiO2. In order to investigate the performance as passive counter-electrode in Li+ based electrochromic devices the films were tested by cyclic voltammetry with in-situ transmission control. Chemical composition was measured by Rutherford backscattering spectrometry. Surface morphology was analyzed by atomic force microscopy.
Hydroxyapatite (HA) is a ceramic material which is intersecting as a coating of Titanium protheses because of its biological compatibility. The characteristics of the HA films deposited by pulsed laser ablation are influenced by a large number of process parameters. After the deposition HA film show also an ageing effect related to changes of the film properties. The knowledge of the refractive index of HA film could help to identify the properties both during the film growth and, after the deposition, during the exposure to different environments. The HA films were deposited on Ti substrates and optically characterized in the 500-850 nm optical spectrum by spectroscopic ellipsometry and the resulting refractive indices are reported in the paper. Starting from the results of SEM and XRD measurements a suitable model was developed that allowed the solution of the ellipsometric equations in terms of n and k over the entire measured spectrum at 65 degrees and 75 degrees incidence angles.
The development of coatings which show a good thermal conductivity plays an important role in the realization of new laser schemes like the Yb:YAG disk laser. We present results on investigations on the thermal conductivity of different thin film materials produced by conventional thermal evaporation and ion beam sputtering. With the optimized IBS-process high reflecting coatings are produced on the end faces of Yb:YAG disk lasers. A significant reduction of the thermal gradient in the laser disk was observed, which is an essential requirement for high power lasers besides the excellent optical quality of the coating.
At the moment WO3 is the most suitable material as cathodic film for electrochromic devices, mainly because of its good mechanical and physical properties and efficiency in coloring and bleaching upon cation intercalation. We studied the effect of the electrochemical insertion of small cations (H+, Li+ or Na+) in WO3 thin films by means of `in situ' transmittance and reflectance measurements. As a result we observed for Li+ and Na+ a behavior depending on the cation concentration: under a given limit (around 14 mC/cm2 for a 100 nm thick film) the optical changes were only related to the number of electrons entering the film from the external circuit for the electric charge neutralization. Beside this limit a phase transition was observed with the formation of the so called tungsten bronze and the electrochemical process was no longer reversible. With regard to the intercalation of H+ beside the above mentioned limit the saturation of the optical density was observed, but no bronze formation was detected even for charge insertion as large as 50 mC/cm2. As a consequence, during the insertion of Li+ and Na+ the current density must not exceed a maximum value that also depends on the diffusion of cations inside the film. For the WO3 samples studied during this work we found the diffusion coefficient for Li+ and Na+ to be related to the cation concentration and varying in the range of 10-9 divided by 10-11 cm2/s.
Photothermal Deflection Spectroscopy (PDS) is a recently developed technique that is finding a useful application in the measurement of low optical absorptance of thin films. Among the noise sources affecting the PDS measurement, probe beam pointing instability and mechanical vibration play a considerable role. In this work an optoelectronic system for the reduction of their influence is described. Moreover, PDS measurements are typically performed keeping the sample immersed in a deflecting liquid; thus measured values of absorptance must be corrected when other surrounding media, as air, are considered. This correction is an easy task for single film coatings. Here the general case of an unknown multiplayer coating is analysed; a range of values containing the true absorptance in air is obtained by theoretical analysis and a practical method to evaluate the absorptance in air is discussed. Finally, deflecting liquids alternative to the commonly used CCI4 have been examined. Useful optical range, thermal diffusivity and “relative deflecting power” of CCI4, CS2, Iso-octane and Aceton are reported.
The optical characterization of thin films is a field of continuous
investigation, nevertheless there is a lack of data of refractive indexes
(n-ik) in the infrared spectrum. In this paper a method for the determination
of the optical parameters n and k is described that starting from
spectrophotometric measurements gives, by computer, the refractive index
profile in a wide wavelength range. Y203, Al203, HfO2, ZnSe and ZnS are the
examined film materials and their n and k values in the visible and infrared
spectrum (0.35-20 m) are reported.