Spectroscopic dielectric tensor of monoclinic crystals: CdWO₄

Generalized ellipsometry measurements were made using 12 orientations of a monoclinic CdWO₄ crystal. 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 the dielectric tensor and determine the birefringence for light passing along the unique axis, but the orientation of the dielectric tensor 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 unique axis is perpendicular to the sample surface, the condition ρ_ps = -ρ_sp is valid. If the unique axis is perpendicular to the plane of incidence, ρ_sp = ρ_ps = 0, and if the unique axis is in the plane of incidence, parallel to the sample surface, then ρ_ps = ρ_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.