## 2 Experimental Setup  
The schematic of the experimental configuration for BGSe-OPO is depicted in Figure 1a. Firstly, the high-energy pump laser system is produced by an electro-optically (EO) Q-switched Nd:YAG oscillator at 1064 nm. The pump laser system can provide the highest output energy of \(\sim\)110 mJ at the PRR of 10 Hz at \(\sim\)10 ns (full width at half maximum--FWHM) pulse duration. It has an approximately Gaussian temporal and spatial profile. Due to the OPO system being configured with a double pump passing cavity, an optical isolator (ISO) behind the pump source was utilized to prevent possible damage to the Nd:YAG oscillator from the reflected pump light. Meanwhile, a combination of a half-wave plate (HWP) and a 45\({}^{\circ}\) polarized beam splitter (PBS) was used to alter input pump energy with vertical polarization into the OPO without changing the pulse width and pump beam spatial profile. Dumper was a black body to absorb the leaked pump energy. In order to avoid damage to the optical modules at the high pulse energy level and allow stable operation for a long time, the pump beam was not focused and a beam shaper (BS) system was employed to reshape and collimate the pump beam to a large spot diameter of about 6 mm (D4\(\sigma\)) in the position of the OPO. The corresponding highest peak on-axis intensity of \(\sim\)35 MW/cm\({}^{2}\), which was much less than the damage threshold of antireflection (AR) coating at BGSe crystal end facts (\(\sim\)80 MW/cm\({}^{2}\)). The design of a large pump beam can curb the influence on the output results from spatial birefring  
Figure 1: (**a**) Schematic diagram of the BGSe-OPO pumped by 1064 nm Nd:YAG oscillator laser. Inset: photograph of the adopted BGSe crystal. ISO: optical isolator; BS: beam shaper; HWP: half-wave plate; PBS: 45\({}^{\circ}\) polarized beam splitter; M1: input plane coupler; OC: output plane coupler; Ge: germanium filter; EM: energy meter. (**b**) The transmittance curve of the uncoated BGSe sample with a thickness of 15 mm. the beams. Consequently, a BGSe crystal with a large cross-section of 8 \(\times\) 8 mm\({}^{2}\) was chosen, cut at \(\theta\) = 47.1\({}^{\circ}\) and \(\varphi\) = 0\({}^{\circ}\) with respect to the \(x\)-\(z\) principal plane for type I phase matching (e \(\rightarrow\) o + o) condition based on the Sellmeier equations (S-E) in [21] to ensure large effective nonlinearity. Furthermore, the effective nonlinear coefficient can be counted with \(d_{\mathrm{eff}}=d_{16}\mathrm{cos}^{2}(\theta)+d_{23}\mathrm{sin}^{2}( \theta)\) in this condition, derived from the symmetry of BaGa\({}_{4}\)Se\({}_{7}\) crystal. Both crystal facets were finely polished and had AR coatings for the pump, signal and idler waves. The reflection loss at each surface was no more than 1% at 1064 nm. In order to obtain large-size, high-quality crystals with few defects, two aspects are improved in the NLO crystal growth process by the Bridgman-Stockbarger method. First, the BGSe crystal was synthesized by high pressure, which can provide more homogeneous polycrystalline raw material with fewer impurities. Second, the crystal growth temperature field was optimized with the use of COMSOL Multiphysics simulation software to help reduce the internal defects of crystals and obtain large-size samples [22]. In addition, the transmission curve of the uncoated BGSe sample with a thickness of 15 mm, in the range of 0.2-20 \(\upmu\)m, is shown in Figure 1b. It presents higher transmission in the range of between 1 \(\upmu\)m and 12 \(\upmu\)m and can extend up to 18 \(\upmu\)m.  
The BGSe-OPO was set up in a linear cavity architecture. Plane mirror M1 with high transmission (HT) at the pump (T \(>\) 94% at 1064 nm) and high reflection (HR) for the signal and idler waves (R \(>\) 98% at 6450 nm and R \(>\) 96% at 1274 nm) was used as the input coupler (IC) of the OPO. The other plane mirror with an HR-coated pump and signal lasers (R \(>\) 98% at 1064 nm and 1274 nm) and AR-coated for the idler (T \(>\) 90% at 6450 nm) was used as an OC of the OPO.