Ultralow Gradient HGF-Grown ZnGeP2 and CdGeAs2 and Their Optical Properties

Abstract

ZnGeP2 and CdGeAs2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients (d36 equals 75 pm/V and 236 pm/V for ZnGeP2 and CdGeAs2, respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP2's transparency range (0.62−13 μm) makes it the optimum material for shifting the wavelength of 2-μm pump lasers into the 3–5-μm range via optical parametric oscillation (OPO), whereas that of CdGeAs2 (2.3–18 μm) is better suited for doubling the frequency of CO2 lasers (9–11 μm) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses.The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a-axis and c-axis thermal-expansion coefficients of ZnGeP2 differ by a factor of two, whereas those of CdGeAs2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.