Spectroscopic Dielectric Tensor of Monoclinic Crystals: CdWO4

Generalized ellipsometry measurements have been made using twelve orientations of a monoclinic CdWO4 crystal, and a formalism for determining the dielectric tensor of this non cubic system in the laboratory reference frame has been developed. Using these measurements and the associated analytical methods presented here, it is shown that the four independent complex elements of the dielectric tensor can be determined at each wavelength. Below the band edge (~4 eV), the dielectric tensor is real, and therefore, it is possible to uniquely diagonalize this tensor and determine the birefringence for light passing along the unique b axis, but the orientation of the axes will be a function of wavelength. Above the band edge, unique diagonalization is not possible. The generalized ellipsometric spectra show some symmetry in the cross polarization coefficients. When the b axis is perpendicular to the sample surface, the condition ps = sp is valid. If the b axis is perpendicular to the plane of incidence, sp = ps = 0, and if the b axis is in the plane of incidence, parallel to the sample surface, then ps = sp 0. The combined experimental and analytical methods described here are applicable to the determination of the spectroscopic dielectric tensors of monoclinic crystals in general.