The new nonlinear crystal for the mid-IR CdSiP 2 was discovered only very recently but the interest in this chalcopyrite is enormous because it possesses most of the attractive properties of the related ZnGeP 2 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 ZnGeP 2 which shows positive

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... ence. We now measured its nonlinear coefficient using SHG of femtosecond pulses generated near 4.6 µm from a seeded KNbO 3 optical parametric amplifier. The SHG efficiency was compared for uncoated samples of CdSiP 2 and ZnGeP 2 , 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 CdSiP 2 and ee-o type in ZnGeP 2 ), which were in good agreement with the existing Sellmeier approximations, we arrived at d 36 (CdSiP 2 )~d 36 (ZnGeP 2 ) which is rather unexpected having in mind the larger band-gap of CdSiP 2 . The reliability of the measurement was tested at the same wavelength by comparing ZnGeP 2 with HgGa 2 S 4 which led to the result d 36 (ZnGeP 2 )~3d 36 (HgGa 2 S 4 ), in very good agreement with previous estimations.