Growth of crack-free ZnGeP2 large single crystals for high-power mid-infrared OPO applications

• More than 500 g of high-purity ZGP polycrystalline material was synthesized in one run.
• A high-optical quality ZGP single crystal with the diameter of 50 mm was reported for the first time.
• The absorption coefficient of ZGP–OPO device was reduced to 0.02 cm−1.

In this paper, more than 500 g of high-purity ZGP polycrystalline material has been synthesized in one run, and high-optical quality ZGP single crystals with dimensions of 50 mm in diameter and 140 mm in length have been also successfully grown by Vertical Bridgman technique. From data of the near-infrared optical absorption coefficient, we found that the large crystal had lower absorption losses at the near-infrared waveband than the small one. Meanwhile, the absorption coefficient of 24-mm length ZGP–OPO devices for o-polarized 2.05 µm light was significantly reduced to about 0.02 cm−1 after thermal annealing and high energy electron irradiation. Moreover, improvement in the polishing of ZGP–OPO devices had resulted in increase of laser-induced damage threshold. The average value of laser-induced damage threshold was increased to 2.0 J/cm2 under the condition of 2.05 µm wavelength, 30 ns pulse width and 10 kHz pulse rate frequency. As a result, the maximum output of 20.5 W was obtained by high pulse repetition frequency Ho:YAG laser emitting at 2.09 µm under a pump power of 46 W. The corresponding slope efficiency was 68.7% and the conversion efficiency was 44.3%. These results manifested that the large ZGP crystals with high optical quality were acceptable for the fabrication of the infrared OPO devices.