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.

A BaGa₄Se₇ nanosecond optical parametric oscillator (OPO) shows extremely wide idler tunability in the mid-IR (2.7-17 μm) under 1.064 μm pumping. The ∼10  ns pulses at ∼7.2  μm have an energy of 3.7 mJ at 10 Hz. The pump-to-idler conversion efficiency for this wavelength reaches 5.9% with a slope of 6.5% corresponding to a quantum conversion efficiency or pump depletion of 40%.

No abstract

Lithium selenoindate (LiInSe 2) is a new nonlinear chalcogenide biaxial crystal, related to LiInS 2 and transparent from 0.54 to 10 µm at the 50% level (10 mm thickness), that has been successfully grown in large sizes and with good optical quality. We report on new physical properties that are relevant for laser and nonlinear optical applications and summarize all relevant characteristics, both from the literature and such measured in the present work. With respect to AgGaS(e) 2 ternary chalcopyrite materials, LiInSe 2 displays a nearly isotropic thermal expansion behavior with 3 to 5-times-larger thermal conductivities associated with high optical damage thresholds, and low intensity-dependent absorption, allowing direct high-power down-conversion from the near-IR, especially 1064 nm, to the deep mid-IR. Continuous-wave difference-frequency generation (5.9-8.1 µm) of Ti:sapphire laser sources is reported for the first time as well as nanosecond optical parametric oscillation with Nd:YAG laser as a pump source at 100 Hz and idler tuning between 4.7 and 8.7 µm.

No abstract