Studies on the crystal distortion -- birefringence relationship in ZnGeP$_{2}$

ZnGeP$_{2}$ as a member of the ternary II-IV-V$_{2}$ compound chalcopyrite family has been extensively studied over the last decades as promising birefringent material for nonlinear optical applications in the near- and mid infrared (IR) wavelength region. The high birefringence in the ZnGeP$_{2}$ material system enables applications such as second-harmonic generation (SHG), optical parametric amplifier (OPO), and sum/difference frequency mixing. The birefringent effect in the ZnGeP$_{2}$ crystal structure is related to the compressive distortion of the chalcopyrite lattice in c-direction. Utilizing unpolarized/polarized Raman spectroscopy (RS) and infrared reflectivity (IR) measurement, we studied the vibrational phonon modes in ZnGeP$_{2}$ crystals and analyzed phonon contributions related to the anisotropic lattice distortions. The effect of birefringence is analyzed from the shift of the $\Gamma_4^{L0}$ Raman mode. Furthermore, the symmetry forbidden Raman (SFR) scattering modes, observed in the ZnGeP$_{2}$ Raman spectra, have been analyzed to understand contributions related to the lattice distortion and contributions related to crystal defects. Our studies show that the non-linear mixing of radiation only occurs along the [001] crystalline plane, where conservation of momentum or so-called phase matching is present.