THE COMBINED ELLIPSOMETRIC METHOD OF COMPLETE OPTICAL CHARACTERIZATION OF CRYSTALS. III. EXPERIMENTAL DETERMINATION OF THE ORIENTATION OF OPTICAL AXIS IN CRYSTAL

Abstract

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, n_eff = f(α). The second variant is based on the fact that the effective refractive index, n_eff, 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, n_eff = 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 N_g 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).

Key words: ellipsometry, optical indicatrix, principal refractive indexes, uniaxial and biaxial crystals.