Zinc germanium diphosohide (ZnGeP₂) belongs to the chalcopyrite family with a direct band gap, and it is a ternary analog of the III-V group binary compound GaP with the zinc-blende structure. However, the chalcopyrite structure is tetragonally distorted from the simpler zinc-blende structure. Due to the distorted structure relative to its binary analogue, ZnGeP₂ exhibits a far wider range of physical properties. ZnGeP₂ has large nonlinear optical coefficient (d₃₆=75 pm·V^-1), a wide transparency range from 0.74 to 12μm, sufficient birefringence (Δn= 0.04) for type I and type II phase matching, and relatively high thermal conductivity (0.35W·cm^-1·K^-1) for high average power applications. All these physical properties make ZnGeP₂ suitable for frequency conversion applications such as tunable mid-infrared optical parametric oscillator (OPO) laser systems. The main goal of the present work is to systematically study and deeply discuss the growth, optical properties and applications of ZnGeP₂.

Firstly, ZnGeP₂ polycrystalline ingots were synthesized by two-temperature method, and the maximum mass is about 470g for one run. Fig.1 shows the synthesized ZnGeP₂ ingots with 470g. The XRD suggests that they show chalcopyrite structure, high pure and good crystallinity. The ingots are suitable for growth as raw materials. Moreover, the study indicates that the more the charge amount, the higher the yield of product.

Fig.1 The synthesized ZnGeP₂ polycrystalline ingots with 470g

Secondly, ZnGeP₂ crystals were grown by Vertical Bridgman technique, and the largest size is about Φ50mm×140mm. Fig.2 shows the as-grown ZnGeP₂ crystals.The XRD and rocking curves suggest that they show good crystallinity and single phase, the XRF indicates that the as-grown ZnGeP₂ is stoichiometric, and the absorption coefficients of as-grown ZnGeP₂ are 0.05-0.08cm^-1 at 2.05µm for o-light. However, the absorption coefficients of annealed and other post-treated ZnGeP₂ reduce to 0.01-0.03cm^-1 at 2.05µm for o-light. The crystals are suitable for frequency conversion applications.

Fig.2 The as-grown ZnGeP₂ crystals    

Finally, the ZnGeP₂ devices were tested for OPO. The ZnGeP₂ crystals were cut to 6mm×6mm×24mm in sizes after thermal annealing. Both surfaces of the crystal were carefully polished and antireflection coated for pump, signal, and idler lights. The ZnGeP₂-OPO devices are shown in Fig.3. Fig.4 shows the ZnGeP₂-OPO experiment setup. The OPO output wavelengths were in the spectral range of 3-5 mm. The results suggest that the output power of 30W is obtained pumping at 2.09µm and the conversion efficiency is 56%. Thus, ZnGeP₂ is an excellent OPO material and is acceptable for the fabrication of the infrared nonlinear optical device

Fig.3 The ZnGeP₂-OPO devices

Fig.4 The ZnGeP₂-OPO experiment setup

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1. Structural and optical properties of ZnGeP₂ single crystals
2. A High-power Generation in Mid-Infrared Region Based on ZnGeP₂ Optical Parametric Oscillation
3. Investigation of Defects in ZnGeP₂ Single Crystals by X-ray Topography on Base of Borrmann Effect
4. The Analysis of Defects in ZnGeP₂ Single Crystals from Birefringence Images
5. Bulk Growth ZnGeP₂ crystals and their Study by X-Ray Topography