**2. Experimental Setup**

The experimental setup of the BGSe-based OPO is shown in Figure 1. A side-pumped electro-optic Q-switched Nd:YAG laser was adopted as the fundamental 1064 nm pump source. The side pump module was composed of five quasi-CW 808 nm laser diode (LD) arrays and a 148 mm long Nd:YAG rod (0.6% doped). The pulse width of the quasi-CW LD arrays was 200 μs and the repetition rate was adjustable in the range of 1–1000 Hz. The electro-optic Pockels cell consisted of two BBO crystals with dimensions of 6 mm × 6 mm × 20 mm. The laser cavity was a plano-plano cavity with a high reflection mirror M1, an output coupler M2, a quarter wave plate and a Brewster polarizer. The output coupler M2 was partial reflection (R ≈ 5%) to avoid laser damage to the crystal rod caused by the high energy intensity in the cavity. The total length of the laser cavity was 350 mm.

A half wave plate and a Brewster polarizer were combined as an energy attenuator to adjust the pump energy and maintained the polarization direction. An optical isolator was employed to avoid the possible reflected damage to the Nd:YAG laser. The DP-SRO BGSe-OPO was composed of two BaF2 mirror M3 (1.064 μm AR and 1.35–1.65 μm and 3.65–4.50 μm HR) and M4 (3.65–4.50 μm AR and 1.064 μm and 1.35–1.65 μm HR). The DP-SRO configuration can effectively improve the mid-infrared output and reduce the threshold [22].

A large-size BGSe crystal (shown in the inset of Figure 1) with dimensions of 8 mm × 8 mm × 15 mm was used as the nonlinear optical crystal in OPO. The BGSe crystal employed was grown by vertical Bridgman method [23]. In addition, two zone annealing technique was used to improve the optical properties. The BGSe crystal exhibits high transmittance from 0.47 to 18 μm and low absorption coefficient of 0.042 cm−<sup>1</sup> at 4 μm. The BGSe crystal was cut at θ = 42.5◦ and ϕ = 0◦ for Type I phase-matching condition (o-ee). Both sides of the BGSe crystal were well-polished and AR-coated for 1.064 μm, 1.3–1.6 μm and 3–5 μm. The crystal was wrapped in indium foil and cooled to 25 ◦C with brass heat sink to reduce the thermal effect in BGSe crystal. The wavelength of the signal wave was measured by a spectrometer (Agilent, 86142B, Santa Clara, CA, USA), which was used to calculated the wavelength of the generated mid-infrared wave. A dichroic BaF2 mirror (3.65–4.50 μm AR and 1.064 μm and 1.35–1.65 μm HR) was used as the long-wave pass filter. The energy of the generated mid-infrared wave was measured by a mid-infrared energy detector (Newport corporation, 919E-0.1-12-25K, Irvine, CA, USA).

**Figure 1.** Schematic of BaGa4Se7-OPO. The inset shows the photograph of the BaGa4Se7 crystal and the transmittance curve (Adapted with permission from Ref. [23]. Copyright © 2019, American Chemical Society).
