Homogeneous growth of bulk BaGa ₄ Se ₇ single crystal and femtosecond laser performance and photodarkening

BaGa 4 Se 7 (BGSe) is a relatively new nonlinear optical (NLO) single crystalline material which has been grown successfully into large single-crystal boules through vertical Bridgman in Russia and China. BGSe shows promise as a down frequency converter for photonics applications with a 1–2 µm pump and conversion from 2–16 µm. In part due to its extremely flat tuning curve for 2 µm pumping, a single phase-matching angle can be used to access a whole mid-wave infrared and long-wave infrared (LWIR) range through relatively shallow angle tuning over a few degrees. BGSe is one material out of a family of barium single-crystal compounds (BaGa 4 S 7 , BGS; BaGa 2 GeSe 6 , BGGSe; BaGa 2 GeS 6 , BGGS; and Ba 2 Ga 8 GeS 16 , B2GGS) which are all capable of NLO frequency conversion. Further, most of these barium compounds seem to demonstrate a higher laser damage threshold (LDT) compared to the widely used LWIR NLO AGSe. For these reasons, especially the broad spectral output, BGSe has seen much attention in the last decade. From a growth perspective, BGSe is one of the easier barium compounds to grow and so is more readily available. In this work, we demonstrate BGSe grown in a horizontal gradient freeze furnace with low thermal gradients (0.5−3 ∘ Ccm −1 ) and growth rates ranging from 0.02 to 0.05cmh −1 to grow single crystalline material. This work demonstrates the capability to grow large BGSe single crystals through an improved synthesis cycle and process control. Via Raman microscopy, we demonstrate a repeating twin-like defect in the crystal which is intrinsic and visible through mapping of the surface with 532 and 785 nm light. We demonstrate femtosecond operation of BGSe crystals as a second-stage OPA and compare to AGS and BGGSe. Finally, we study permanent photodarkening which is observed near but below LDT; however, it does not absorb above 1 µm and so does not impact pump, signal, or idler transmission.