In this paper, we present the optical and nonlinear optical properties of a new nonlinear crystal CdGeAs2. The chalcopyrite structure (4̄2 m) crystal is positive birefringent with Δn equal to 0.1. It is phasematchable for second harmonic generation (SHG) and parametric interactions over most of its transparency range from 2.4 to 18 μm. In addition, it has the largest known nonlinear coefficient for a phasematchable crystal with the exception of tellurium. The measured nonlinear coefficient is 3.4 times GaAs. The calculated doubling efficiency for SHG of a CO2 laser is 10 times that of tellurium. The crystal promises to be a very useful nonlinear material for parametric interactions between 3 and 18 μm.

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