Two new mid-IR NLO materials - barium thiogallate (BaGa4S7) and barium sellenogallate (BaGa4Se7) – have recently been reported to exhibit notably wide band gaps (3.54 eV and 2.64 eV repectively), deep infrared transparency (cut-off at 13.7 m and 18 m respectively), moderately high nonlinear coefficients (5 pm/V and 20 pm/V), and favorable growth of high-optical-quality crystals from stoichiometric melts. To evaluate this final claim, and to establish a viable commercial source for these materials, we investigated the growth of these compounds in transparent furnaces using the Horizontal Gradient Freeze (HGF) technique. The compounds were synthesized from high-purity elemental starting materials by direct synthesis in carbon-coated boats in fused-silica ampoules. Self-seeded, crack-free single crystals (up to 19-mm diameter x 50-mm long) were in fact grown in a single attempt with good optical quality. FTIR and VIS/NIR spectral measurements confirmed the low losses and wide transparency range of these materials. Orientation and fabrication of device crystals (aided by facets on the as-grown boules) is in progress.
Peter G. Schunemann and Kevin T. Zawilski, "Horizontal Gradient Freeze Growth of Wide Band Gap Mid-Infrared NLO Crystals BaGa4S7 and BaGa4Se7 (Conference Presentation)," Proc. SPIE 10516, Nonlinear Frequency Generation and Conversion: Materials and Devices XVII, 105160T (Presented at SPIE LASE: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291370.5751428848001.
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