Gerard Albrand

Comparison of the properties of titanium dioxide films prepared by various techniques.

Fourteen university, government, and industrial laboratories prepared a total of twenty pairs of single-layer titanium dioxide films. Several laboratories analyzed the coatings to determine their optical properties, thickness, surface roughness, absorption, wetting contact angle, and crystalline structure. Wide variations were found in the optical and physical properties of the films, even among films produced by nominally the same deposition techniques.

Refractive index and inhomogeneity of thin films

The fact that the optical characteristics of thin-film materials are generally different from those of the same materials in bulk form is well known. The differences depend very much on the conditions in which the deposition has been carried out. A good understanding of these differences, their causes, and the influence of deposition parameters is vital if we are to be able to improve coating quality. We have developed two complementary methods with the objective of deriving information on the index of refraction and its variation throughout the thickness of the film. Perceptible optical inhomogeneity is normally present and appreciable inhomogeneity is frequently present in thin films. Such inhomogeneity is usually associated with layer microstructure. The first is a postdeposition technique that makes use of measurements in air of the transmittance and reflectance of the layer under study over a wide wavelength region. The second, in contrast, makes use of in situ measurements, that is measurements made under vacuum and during the actual deposition of the layer. We shall show with the help of several examples that the two methods lead to results that are consistent and demonstrate the existence in deposited materials of an inherent variation of the index of refraction normal to the surface. The thermal sensitivity of the layer properties and their tendency to adsorb atmospheric moisture must be taken into account before the residual differences between the two techniques can be explained.

Theory and application of antiscattering single layers: antiscattering antireflection coatings

We recall the analytical expression that gives, for a rough surface illuminated at normal incidence, the light scattered in the half-space containing the specular reflection direction. Two cases are studied: the bare substrate and the substrate coated with one transparent layer. It is shown, for this specular direction, that the light scattered from a single layer can be equal to zero (perfect antiscattering) in certain conditions relative to the roughnesses of the two layer interfaces. Data calculation proves that this antiscattering effect occurs in all directions of the half-space of the reflected light. The experimental results are in good agreement with this theoretical analysis for five different dielectric materials. This study brings out most information about the grain of the material, which is responsible for the residual roughness.

In situ and air index measurements: influence of the deposition parameters on the shift of TiO2/SiO2 Fabry-Perot filters

We measure the refractive index of thin films of TiO2 and SiO2 for given deposition parameters. Two complementary methods are used. The first is a postdeposition technique which uses the measurements of reflectance and transmittance in air. The second, in contrast, makes use of in situ measurements (under vacuum and during the actual deposition of the layer). The differences between the values deduced from the two methods can be explained by the amount of atmospheric moisture adsorbed by films. One tries to minimize these shifts for the two materials by choosing deposition parameters. The difficulties come from the absorption losses which must be as small as possible. We use the measured refractive indices of individual layers to give good numerical prediction of the wavelength shift (observed during the admittance of air after deposition in the vacuum chamber) of the transmittance peak of multidielectric Fabry-Perot filters.

Surface optical coatings by ion assisted deposition techniques: study of uniformity

Uniformity of layers produced by IAD strongly depends on the characteristics of the ion beam: optical characterization is made by guided wave measurements. The thickness and refractive index including anisotropy must be considered separately to interpret the measured uniformity.

Guided-wave characterization techniques for the comparison of properties of different optical coatings

The specific behavior of optical thin films very often leads to limitations of optical system performance. Accurate characterization techniques for evaluating film properties are necessary to understand this behavior. Characterization techniques based on the propagation of guided waves in the thickness of the films appear to be very useful. We report our particular way to determine the refractive index and the thickness of both isotropic and anisotropic thin films. Guided-waves techniques are sensitive enough to detect slight variations of thin film optical constants, so we use them to study the variations of refractive index versus temperature. From this we can obtain the thermorefractive coefficients ∂ n /∂ T of our layers. Moreover, we can obtain, in some cases, the nonlinear refractive index coefficient. We also measure guided-wave attenuation and laser damage threshold with a digital imaging system. These means, dependent on guided waves, are used in combination for a comparative analysis of TiO 2 and Ta 2 O 5 layers made by different eposition techniques (conventional evaporation, ion assisted deposition and ion plating).

Synthesis and research of the optimum conditions for the optical monitoring of non-quarter-wave multilayers.

Many optical filtering problems require the use of assemblies of layers with thicknesses that bear no obvious relationship to each other. Here we present the results obtained for a number of examples in which optical monitoring is performed with a change of control wavelengths for each layer of the stack. For this, it is necessary to determine for each layer the different wavelengths that provide an extremum of transmittance when the required thickness is achieved. We show that this leads, in some cases, to making the benefit of error compensation analogous to the well-known method used in the production of quarter-wave stacks. Because ion-assisted deposition and ion-plating techniques are suitable from the point of view of refractive-index reproducibility, optical monitoring can be used at a good level of performance. However, the production of high-quality optical thin films needs more than just the choice of a monitoring process. In particular, problems of uniformity are critical for high-performance coatings. Here we show how uniformity can be determined for each material involved.

Achromatic damage investigations on mirrors for UV-free electron lasers

Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localized spatial modification induced by the non-uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness.

Toward resistant UV mirrors at 200 nm for free electron lasers: manufacture, characterizations, and degradation tests

Storage Ring Free Electron Laser are attractive and full of promise tunable and powerful laser sources for the UV range. Concerning the optical cavity, the relatively small gain obtained in the UV calls for the necessity to use high reflectivity multilayer mirrors with reliable longevity in synchrotron environment. It is also crucial to limit their absorption in order to optimize the extracted power required for most of the applications. Indeed, the front mirror of the laser cavity receives not only the first harmonic where the lasers operates but all the synchrotron radiation emitted by the undulator: a wide spectrum extending towards X rays. These short wavelengths are responsible for the mirror degradation which results from changes in the coating materials (high induced absorption, color centers, heating...) as well as from carbon contamination due to cracked hydrocarbons originating from the residual vacuum atmosphere. Deposition technologies which allow the manufacture of very dense oxide coatings with low absorption and high reflectivity in UV spectral region were optimized and characterized for this purpose. We report here degradation studies performed on UV mirrors for Storage Ring Free Electron Lasers down to wavelengths as short as 200 nm.

Optical materials in powder forms: characterization techniques

For a large number of specific applications, optical materials must be used in powder forms. In this context it has become highly necessary to characterize the optical and microstructural properties of such powders. Preliminary scattering experiments have shown that the scattering intensity from materials in powder forms could be quite different than that of the same homogeneous materials. In particular we have noted a strong increase in the level of the scattered light that could come from the bulk scattering. Here we show how to use the light scattering techniques in order to separate and determine the roughness and inhomogeneities of the samples. The same techniques are used to determine with accuracy the refractive index and absorption of the powders. In a second step, the same powders are evaporated in thin film forms, and we use classical spectrophotometric techniques to determine their refractive index and dispersion laws. The result are compared and discussed with those obtained with light scattering. In a general way, this study involves surface and bulk theories of light scattering, together with angle- resolved measurements, and atomic force microscopy.