BIREFRINGENCE AND REFLECTIVITY OF SINGLE-CRYSTAL CDAL2SE4 BY GENERALIZED ELLIPSOMETRY

Transmission and reflection generalized variable-angle spectroscopic ellipsometry and polarized transmission intensity measurements over the photon energy range from 0.74 to 5.0 eV have been used to characterize the optical properties of the ordered-vacancy compound ${\mathrm{CdAl}}_{2}{\mathrm{Se}}_{4}.$ We report the dispersion of the uniaxial refractive index below the band gap. The onset of weak absorption indicates the fundamental band edge at about 2.95 eV, but does not reveal the nature of the lowest band-to-band transition. Above the fundamental band gap we assign four possible critical-point structures from a line-shape analysis of the sample dielectric function.