Quasiparticle effects on the linear and nonlinear susceptibility of ZnGeP.

The semiconductor zinc germanium diphosphide (ZnGeP) has wide applications in the infrared nonlinear optics (NLO) due to its high nonlinear optical coefficient, wide infrared transparency range and high thermal conductivity. Absorptions near the pump or generation wavelength limit the effectiveness of this materials, with their complicated microscopic origins remaining largely elusive. Most research on the absorption mechanism of ZnGeP focused on the defect effect, while the quasi-particle effect and exciton effect are significant as well. We herein carried out the studies of the electronic band structure and optical properties of ZnGeP crystal. The quasiparticle and excitonic effects were examined by comparing the results of PBE, GW approximation and Bethe-Salpeter equation. Quasiparticle effect was found to widen the quasi-direct band gap and increases the valence and conduction band dispersions, which mainly blue-shifts the imaginary part of the dielectric function. The increased band gap also leads to a broadened lineshape in the second order susceptibility. The excitonic effects significantly enhance the peak intensity in the long wave regime of the dielectric function and red-shift the peaks in the high energy regime, leading to the greatly improved agreement with experiment. Our results provided a microscopic guidance for improving ZnGeP's optical performance.