Birefringence of optically uni-axial ternary semiconductors

Refractive indices and birefringences for some representative uni-axial crystals such as II- SiP2 and II- GeP2 have been determined over a wide range of wavelengths by the use of first-principles electronic structure calculations. First, the calculated refractive indices are fitted usually by a generalized Sellmeier equation which consists of several oscillation terms involve in its parameters more direct information about material such as electronic transitions or resonance wavelengths. Then, in contrast to all other semiconductors under discussion our spectra show a negative birefringence for CdSiP2 in agreement with the experimental data, and they exhibit a considerable dispersion near the band gap.