Phase‐matching properties of BaGa₄S₇ and BaGa₄Se₇: Wide‐bandgap nonlinear crystals for the mid‐infrared

Abstract

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.

Growth and characterization of BaGa4Se7 crystal

“…4, the BaGa 4 Se 7 crystal exhibit high transmittance of nearly 65% in the range of 1-14 mm. Considering the refractive loss estimated with Fresnel equation R¼(n À 1) 2 /(n þ1) 2 (n ¼refractive index) and the reported refractive index of BaGa 4 Se 7 [17], this high transmittance is close to the theoretical limit of (1 À R) 2 , which further demonstrate the good quality of the obtained crystal. The absorption edge in the visible range measured on the single crystal is 0.47 mm, which agrees well with our former study using powder diffuse reflectance method.…”

“…Pure polycrystalline BaGa 4 Se 7 was synthesized from the stoichiometric reaction of the constitute elements with a heating profile that was slightly modified based on the report by Petrov et al [17]. In a typical reaction, 6.866 g Ba (0.05 mol, 99%), 13.945 g Ga (0.2 mol, 99.9999%), and 27.636 g Se (0.35 mol, 99.9999%) were loaded in a fused-silica tube, under an Ar atmosphere in a glove box.…”

“…Besides, it is phase matchable in the entire transparent range (0.47-18 mm) based on theoretical calculations. Shortly after our initial report, Petrov et al [17] obtained bulk crystals of BaGa 4 Se 7 by BridgmanStockbarger technique and measured the refractive index. The experimental refractive index values proved that BaGa 4 Se 7 is phase matchable in the IR range.…”

Growth of CdSiP2 single crystals by self-seeding vertical Bridgman method

“…For example, the smaller alkali ion Li þ was used to substitute for the larger Ag þ in AgGaS 2 and AgGaSe 2 to obtain LiGaS 2 and LiGaSe 2 series crystals [9][10][11][12]. The alkaline earth metal Ba 2 þ was also selected to substitute for the larger Ag þ in BaGa 4 S 7 and BaGa 4 Se 7 [13][14][15]. On the other hand, GeS 2 or GeSe 2 can be added to AgGaS 2 or AgGaSe 2 , respectively, to form AgGaGeS 4 and AgGaGe 5 Se 12 quaternary compounds [16][17][18][19][20].…”

Temperature-Dependent Sellmeier Equations of IR Nonlinear Optical Crystal BaGa4Se7

“…The crystal belongs to the monoclinic space group Pc. It possesses intriguing overall properties for IR NLO applications including wide transparent range (0.47 to 18 µm), suitable birefringence, large nonlinear optical coefficients, and high laser damage threshold [8][9][10][11][12]. The outstanding properties of BaGa 4 Se 7 have been exemplified by some recent laser experiments [13][14][15][16][17].…”