Emile Pelletier

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.

Multiple determination of the optical constants of thin-film coating materials

The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3 refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively.

Automatic determination of the optical constants of inhomogeneous thin films

The refractive index of a layer is a sensitive function of the preparation conditions. Normal incidence measurement of the optical properties can reveal possible inhomogeneity of index. We propose a method of automatic determination of the complex refractive index and thickness of a layer which includes systematic measurement of the degree of inhomogeneity which is represented by a simple model. The usefulness of the technique is demonstrated by examples that form part of an experimental study of a number of useful optical materials including Y(2)O(3), TiO(2), MgF(2), HfO(2), and SiO(2). The dispersions of the refractive index, the extinction coefficient, and of the inhomogeneity are represented by Cauchy formulas with accurately determined coefficients. The results can therefore be readily used in computing the optical properties of thin-film multilayers.

Nd:MgO:LiNbO 3 waveguide laser and amplifier

We report efficient operation of a channel waveguide laser and a channel waveguide amplifier in Nd:MgO:LiNbO(3). For the laser a cw output power of 2.9 mW was obtained for 23.6 mW of absorbed pump power. The absorbed pump power at threshold was 1.5 mW, and a slope efficiency of 13% was achieved. For the amplifier a small-signal gain of 7.5 dB was achieved for 22 mW of coupled pump power.

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.

Anisotropy in thin films: modeling and measurement of guided and nonguided optical properties: application to TiO 2 films

The main purpose of this research is to study the anisotropic behavior of dielectric material in thin-film form. First we present a theory based on a 4 × 4 transfer matrix linking tangential components of the electromagnetic field on one interface to the tangential components of the electromagnetic field on the other interface of an anisotropic thin film. A biaxial model is associated with the columnar structure of the layer. The comparison between measurements of the transmission in normal incidence in cross-polarized light and of guided-mode propagation constants with the calculations allows us to study the biaxial behavior of TiO(2) films. The excellent consistency between measurements and computations demonstrates the validity of the model based on the columnar structure.

Characterizations of optical surfaces by measurement of scattering distribution

The micropolish of good quality optical surfaces can be characterized by measuring the scattered light distribution. Very often the surface defects are not isotropic but display preferred orientations that are translated into an anisotropy of the scattered light distribution. The total amount of light scattered by very high quality surfaces, coated or uncoated, scarcely exceeds a few hundred parts per million. Precise measurement of the distribution of the scattered light is always a task requiring great care and attention to detail. The apparatus is described. All the necessary degrees of freedom have been included so that the scattering may be completely analyzed. It is possible to make measurements out of the plane of incidence so that the complete spatial distribution of the scattered light can be obtained, whatever the angle of incidence of the primary beam. Thus to characterize the geometry of the system we use four fundamental parameters: the angle of incidence i, the two angles θ and ϕ that define the scattering direction, and the angle α that defines the orientation of the scattering surface in its own plane. Only two free parameters need exist because the surface roughness itself, which is the source of the scattered light, only depends on two variables. We have verified experimentally the validity of the relationships linking i, θ, ϕ, and α. In these relationships the expression for the intensity scattered in a particular direction (θ,ϕ) for an uncoated surface at angle of incidence i can be written in the form of the product of a coefficient, depending only on illumination and observation conditions, and of the 2-D Fourier transform of the autocorrelation functions of the surface roughness. Experimental measurements with uncoated surfaces of black glass have accorded with the theory. When the surfaces are coated with one or several layers the problem is more complicated, but it should be possible to derive information on the autocorrelation functions of each of the interfaces and the degree of correlation between them.

Measurements of absorption losses in TiO 2 films by a collinear photothermal deflection technique

We review the principle of photothermal deflection for measuring absorption losses in TiO(2)films. A collinear arrangement gives the best sensitivity for the detection of losses in a low absorbing film deposited on a transparent substrate. The nineteen TiO(2) films produced by six different processes (electron beam evaporation, ion assisted deposition, ion beam sputtering, ion plating, ...), discussed at the 1986 Optical Society of America annual meeting, are measured by this technique. The extinction coefficients of the different films do not show obvious correlation with the deposition method. An important fact is that we have detected a variation in absorption as a function of time on some layers. This absorption shift is connected with the illumination conditions of the sample under study (wavelength: 600 nm; incident power: 400 W/cm(2)). Experimental results over time are given. The evolution of the photothermal signal is different from one sample to another. This phenomenon is partially reversible and depends on moisture degree of atmosphere.

Optical characterization of thin films by guided waves

The study of guided waves in a thin layer allows a precise characterization of refractive index and thickness. Optical anisotropy can also be measured. We show how this technique can be applied to the characterization of a multilayer structure.

Description of a scattering apparatus: application to the problems of characterization of opaque surfaces

We show how the complexity of a micropolished optical surface can be investigated in detail by measurement of the distribution of scattered light. We deal with problems of roughness anisotropy and uniformity together with cleaning problems. Experimental results concern numerous black glasses from different polishing shops and allow a determination of the polish inhomogeneity in a same glass set. After that, we present a detailed study of the apparatus function of the scatterometer, and we determine the limits of validity of our optical characterization method.