The new nonlinear crystal for the mid-IR CdSiP₂ was discovered only very recently but the interest in this chalcopyrite is enormous because it possesses most of the attractive properties of the related ZnGeP₂ but allows in addition pumping at 1064 nm without two-photon absorption and uncritical phase-matching for 6 μm generation with maximized effective nonlinearity. The last feature is due to the fact that this crystal is negative uniaxial in contrast to ZnGeP₂ which shows positive birefringence. We now measured its nonlinear coefficient using SHG of femtosecond pulses generated near 4.6 μm from a seeded KNbO₃ optical parametric amplifier. The SHG efficiency was compared for uncoated samples of CdSiP₂ and ZnGeP₂, both ≈0.5 mm thick, in the low conversion limit (<10% internal conversion efficiency) which justifies the use of the plane wave approximation. Taking into account the experimentally determined phase-matching angles for type-I SHG (oo-e type in CdSiP₂ and ee-o type in ZnGeP₂), which were in good agreement with the existing Sellmeier approximations, we arrived at d₃₆(CdSiP₂)~d₃₆(ZnGeP₂) which is rather unexpected having in mind the larger band-gap of CdSiP₂. The reliability of the measurement was tested at the same wavelength by comparing ZnGeP₂ with HgGa₂S₄ which led to the result d₃₆(ZnGeP₂)~3d₃₆(HgGa₂S₄), in very good agreement with previous estimations.

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