The Nonlinear Coefficient d_(36) of CdSiP_2

摘要

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 birefringence. 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_2S_4 which led to the result d_(36)(ZnGeP_2)～3d_(36)(HgGa_2S_4), in very good agreement with previous estimations.