This part of the article describes the second stage of the combined ellipsometric method of complete optical characterization of crystals. The testing of the second stage was carried out on crystals of lithium niobate (LiNbO3) and cadmium tungstate (CdWO4). The obtained results of measurements of an optically uniaxial LiNbO3 crystal fully confirmed the correctness of the proposed method and its applicability for optical characterization of crystals. In particular, the values of the principal refractive indices {no = 2.280(±0.003), ne = 2.202(±0.002)} and birefringence {Δn = -0.0775(±0.0015)} of the LiNbO3 crystal are in good agreement with the values of these quantities, obtained by other researchers by other methods. Studies of the optically biaxial CdWO4 crystal were important for analyzing the accuracy of determining the optical constants. For generality of this analysis, measurements were performed in different measurement configurations at several angles of incidence of the laser beam. In particular, according to the results of measurements in two configurations (angle of incidence 45°), the following values of the principal refractive indices of the CdWO4 crystal (doped from the melt 0.375 wt.% PbO) were obtained: ng = 2.249±0.002, nm = 2.185±0.002, np = 2.130±0.008. Based on these values of ng, nm, and np, the angle between the optical axes and the optical sign of the crystal were determined. It has been shown experimentally that the CdWO4 crystal is a pronounced biaxial crystal with an angle between the optical axes close to 90°. Possible ways to improve the accuracy of determining the optical constants of crystals are also analyzed.

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In the third part of the article, devoted to the description of the combined ellipsometric method for the complete optical characterization of crystals, two variants of the technique for experimentally determining the orientation of optical axes using ellipsometry are presented. The first variant is based on the search for the circular cross section of the optical indicatrix. As is known, this cross section is perpendicular to the optical axis of the crystal. This is done by successive measurements of the dependence of the effective refractive index on the angle of rotation of the crystal around the normal to the plane under study, neff = f(α). The second variant is based on the fact that the effective refractive index, neff, does not depend on the angle of incidence φ of the beam on the plane under study, if the angle between the incident beam and the optical axis does not change. Thus, if measurements are performed on the plane of the optical axes of the crystal, then the problem is reduced to finding a plane that is perpendicular to the optical axis. This is done by successive measurements of the dependence of the effective refractive index on the angle of incidence of the beam on the plane of the optical axes, neff = f(φ). The second variant of this technique was tested by ellipsometric measurements of a CdWO4 crystal made on the plane of the optical axes. It has been established that the angle between the optical axes (the bisector of the angle is the principal axis Ng of the optical indicatrix) is equal to 2V = 92° ± 1°. Since 2V > 90°, the CdWO4 crystal is an optically negative crystal (the optical sign is minus).

In the fourth part of the article, the application of the proposed method for the complete optical characterization of crystals to optically uniaxial crystals is considered. The object of the study was lithium niobate (LiNbO3) crystals. Refining the orientation of the optical indicatrix in the samples under study, we confirmed the conclusion by direct measurements that for the implementation of the proposed combined ellipsometric method, knowledge of the crystallographic orientation of the crystal under study is completely optional. The obtained values of the optical constants of the undoped LiNbO3 crystal [no = 2.280(±0.003), ne = 2.202(±0.002), Δn = -0.0775(±0.0015)] fully confirmed the correctness of the proposed method and its efficiency in the investigation of uniaxial crystals. The sensitivity of the combined ellipsometric method was tested on LiNbO3 crystals subjected to high-temperature annealing in an H2O atmosphere. The optical constants obtained for an undoped LiNbO3 crystal [no = 2.2455(±0.0015), ne = 2.1965(±0.0015), Δn = -0.049(±0.001)] and a magnesium-doped LiNbO3 crystal [no = 2.2445(±0.0015), ne = 2.1756(±0.0007), Δn = -0.069(±0.001)] is significantly less than the optical constants of the as grown crystal. It is assumed that the main reason that caused such significant changes is hightemperature annealing in an H2O atmosphere. Our main goal was to show the applicability of the method to the analysis of possible changes in the optical constants of a crystal caused by the influence of various factors on its properties. The results obtained show that the goal has been achieved.

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