Calculated and measured reflectivity of ZnGeP2

Reflectivity spectra of the chalcopyrite ternary compound ZnGe${\mathrm{P}}_2$ are studied experimentally and theoretically. The measurements of the reflectivity in both the complete parallel and perpendicular polarizations are made at 5°K. A full zone energy-band structure, the reflectivity and imaginary part of the frequency-dependent dielectric function are calculated using the empirical pseudopotential method. Comparison is made with the measured reflectivity and modulated reflectivity and prominent features in the experimental spectra are identified and associated with interband transitions. In addition, spin-orbit interactions are included at a few points of the Brillouin zone.