Non-oxide nonlinear crystals can be used for frequency conversion of high-power solid-state laser systems operating near 1 µm (e.g. Nd:YAG at 1064 nm) to the mid-IR above ∼5 µm, the upper wavelength cut-off limit of oxide materials. In order to avoid two-photon absorption, the bandgap of such crystals should correspond to <532 nm, a condition which is met by only few chalcogenide compounds. The most prominent of them is the commercially available chalcopyrite type AgGaS 2 (AGS). Other crystals with relatively wide bandgap are the related defect chalcopyrite HgGa 2 S 4 , which is extremely difficult to grow, the orthorhombic LiGaS 2 , LiInS 2 , LiGaSe 2 and LiInSe 2 which exhibit relatively low nonlinearities, the recently developed chalcopyrite CdSiP 2 , which is highly nonlinear and noncritically phase-matchable but transparent only up to ∼6.5 µm, as well as some solid solutions whose composition is almost impossible to control during growth [1,2]. Two new compounds can be now added to this short list, BaGa 4 S 7 (BGS) and BaGa 4 Se 7 (BGSe), and as it will be shown here, both exhibit phase-matching capability to cover the mid-IR spectral range by down-conversion of 1064 nm laser radiation.The acentric orthorhombic structure of BGS was identified as early as in 1983 [3]. More recently, the BaS-Ga 2 S 3 binary phase diagram was studied [4] and single crystals of BGS were grown by the Bridgman-Stockbarger technique [5]. The SHG effect was confirmed by the Kurtz powder test [5]. The bandgap is at ∼350 nm (3.54 eV) and the transparency extends up to 13.7 µm at the 0-level [5] but no information exists on the dispersive properties of BGS.We successfully grew BGS and for the first time its selenide analogue, BGSe, by the Bridgman-Stockbarger method in sizes sufficiently large to measure the dispersion of the refractive indexes, a prerequisite for the prediction of the phase-matching properties. The raw materials used to synthesize the charge were with high purity, 6 Ns for Ga, S and Se, and 99% for Ba. Ba is chemically active, hence glass-carbon containers, evacuated to a residual pressure of 2 × 10 -5 torr, were used for the synthesis. The temperature in the synthesis furnace was initially raised to 1150 °C at 200 °C/h and the charge was held at this temperature for a few hours for homogenization, after that the oven was switched-off to cool the charge down to room temperature. Then the charge was loaded into quartz ampoules of ∅ 18 × 150 mm size which were evacuated again to residual pressure of 2 × 10 -5 torr and inserted into the heating zone of the growth furnace. The temperature was raised to 1130-1140 °C (BGS) and 1070 -1080 °C (BGSe) and after 3 h the ampoule was lowered into the crystallization Biaxial BaGa 4 S 7 and BaGa 4 Se 7 crystals transparent in the mid-IR have been grown by the Bridgman -Stockbarger technique in sufficiently large sizes and with good optical quality to measure the refractive indices and analyze phase-matching properties.
