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摘要
Significance: Mid-far infrared (IR) laser is widely used in fields such as IR remote sensing, laser communication, medical detection, and semiconductor processing. IR nonlinear optical (NLO) crystal can convert existing near IR lasers (12 μm) to mid-far IR range (320 μm) through frequency down-conversion technologies, such as optical parametric oscillation (OPO) and optical parametric amplification (OPA). Therefore, exploring mid-far IR NLO crystals that can realize continuous frequency conversion is essential and imminent. Currently, the commercially available IR NLO crystals are merely restricted to some chalcopyrite-type semiconductors, such as AgGaS2 (AGS), AgGaSe2 (AGSe), and ZnGeP2 (ZGP). It will take years of development to obtain some high-quality crystals and devices. However, some inherent performance defects limit their application in laser frequency conversion. For example, the laser damage thresholds of AGS and AGSe are only 25 MW/cm2 and 11 MW/cm2, respectively, under 1.06-μm wavelength and 35-ns pulse width; hence, they cannot be pumped by high-power laser. Moreover, ZGP cannot use the conventional 1.064-μm laser (Nd: YAG) as the pump source because of its strong absorption in the range of 12 μm. New IR NLO crystals with excellent comprehensive properties are urgently needed to meet the development requirements of mid-far IR lasers. In 2010, we had first discovered and reported BaGa4Se7 (BGSe) as a new IR NLO crystal. BGSe is a positive biaxial crystal that crystallizes in the monoclinic Pc space group (No. 7), it owns a three-dimensional network structure composed of parallelly aligned GaSe4 tetrahedra with Ba2+ in the interstices, and has a large band gap (2.64 eV); wide transmission range (0.4718 μm); large nonlinear coefficient (d22=24.3 pm/V, d23=20.4 pm/V, xyz frame); moderate birefringence (Δn=0.06 at 2 μm); and high laser-damage threshold. It can be pumped by a 13-μm light source to produce up to 18 μm of tunable IR laser. Unlike ZGP, BGSe can obtain large-aperture devices without needing to apply electron irradiation treatment. Recently, the BGSe crystal has attracted extensive attention in the terms of crystal growth and processing, property characterization, and laser frequency conversion from researchers at home and abroad. The obtained experimental results explicitly show that the BGSe crystal has good application prospects in high-power and wide-band IR laser frequency conversion via OPO, OPA, and various differential frequency (DFG) methods (e.g., intracavity/out of cavity and continuous/pulse pumping). Hence, summarizing the recent research results of property characterization and laser frequency conversion is very essential and helpful in mastering the future research direction for the BGSe crystal. Progress: In the terms of property characterization, the experimental results obtained by Yao et al. in 2010 showed that the BGSe crystal has a high transmittance that ranges from 0.47 μm to 18 μm, but an absorption peak at 15 μm. Yao et al. also reported the calculated nonlinear coefficients of d11=18.2 pm/V, d15=-15.2 pm/V, d12=5.2 pm/V, d13=-20.6 pm/V, d24=14.3 pm/V, and d33=-2.2 pm/V under the XYZ frame. Subsequently, in 2015, Zhang et al. measured partial nonlinear coefficients using the Maker fringe method, but failed to obtain the relative symbol. Thereafter, Boursier, Boulanger, Kostyukova, and Guo. also studied the magnitude and the relative sign of the nonlinear coefficients of BGSe. Table 3 summarizes the nonlinear coefficients of BGSe measured by different research groups. In 2012, Yao et al. measured the damage threshold of BGSe under Nd: YAG laser (5 ns, 1 Hz, D=0.4 mm), obtaining a value of 557 MW/cm2, which is 3.6 times that of AGS (155 MW/cm2). Since then, the research groups at home and abroad have further studied the laser-damage threshold of BGSe under different test conditions (Table 4). In the application of laser frequency conversion, Yang et al. realized the IR laser frequency conversion based on the BGSe crystal for the first time in 2013. Thereafter, IR laser frequency conversion experiments based on the BGSe crystal were successively performed. Recently, the representative results include the following: 21.5 mJ at 3.8 μm and 1.05 mJ at 11 μm laser realized by 1.064 μm, 10 Hz pumping source under Type I angle phase matching; 5.12 W and 0.31 W at 4.3 μm and 9 μm laser realized by 2.09 μm, 500 Hz pumping source under Types I and II angle phase matching, respectively; 3.5 mJ at 5.03 μm realized by 2.79 μm, 10 Hz pumping under Type I angle phase matching; and IR laser in the range of 3.157.92 μm produced by differential frequency generation using 1.064 μm and Ti: sapphire laser under Type I angle phase matching. The latest laser experimental results obtained in 2020 include: a coherent broadband mid-IR continuum spanning from 6 μm to 18 μm that was obtained by Zhang et al. using a Cr: ZnS laser system; and the temperature tuning of BGSe first reported by Kong et al. with a temperature range of 30 ℃ to 140 ℃ and an idler light wavelength of 3637 nm to 3989 nm. Conclusions and Prospects: The BGSe crystal is a new type of wide-bandgap IR NLO material with unique advantages and good application prospects in mid-far IR laser output via the frequency down-conversion process. The future research direction of BGSe mainly focuses on improving the crystal quality, searching for the optimal phase-matching direction, and solving the matching problem between the different types of pump sources and the optical properties of BGSe.
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