Vertical Bridgman growth and characterization of large ZnGeP₂ single crystals

The growth of ZnGeP₂ crystals by seeded Vertical Bridgman method was studied. High-quality near-stoichiometric ZnGeP₂ single crystals obtained were of 2030 mm in diameter and 90120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP₂ crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was ∼0.10 cm ^-1 at 2.05 μm, and ∼0.01 cm^-1 at 38 μm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP₂ samples of 6 mm×6 mm×15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 35 μm when the incident pumping power of 2.05 μm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively.