Tunable coherent mid-infrared（MIR）laser sources are promising for nonlinear optics applications owing to the scarcity of direct-emission conventional laser sources in the associated spectral region（3‒20 μm）. MIR radiation is urgently demanded in scientific and technological applications，such as remote sensing，spectral analysis，and material diagnosis^［1］. Currently，a parametric frequency down-conversion process based on nonlinear optical crystals is an effective method for generating widely tunable MIR radiation. Non-oxide nonlinear crystals can be used to perform frequency conversion on solid-state laser systems operating near 1 µm to MIR above 5 µm，which is the upper limit of the wavelength cutoff for oxide crystals such as AgGaQ₂（Q=S，Se），ZnGeP₂（ZGP），and CdSiP₂（CSP）^［2-4］. However，AgGaQ₂ has a low laser damage threshold^［5］. Meanwhile，ZGP must be pumped using ~2 μm lasers to avoid two-photon absorption^［6］，and the infrared（IR）cutoff edge of CSP is located at ~9.5 µm，which limits its application in the long-wave infrared（LWIR）region^［7-8］. In addition，new crystals BaGa₄S₇（BGS）^［9-10］ and BaGa₄Se₇（BGSe）^［11］ developed in recent years are not widely applicable despite their excellent properties. LiSQ₂（S=In，Ga；Q=S，Se）are a series of MIR nonlinear optical materials with promising applications owing to their wide band gap，weak photon absorption，high thermal conductivity，and small thermal-expansion anisotropy. Researchers have successively grown these crystals and investigated their physical properties^［12-14］. An autoclave synthesis method was developed to significantly improve the synthesis quality and efficiency^［15-16］. Frequency down-conversion techniques based on LISe crystals have been reported over the last decade. Tunable nanosecond LISe-OPO lasers at wavelengths of 4.7‒8.7 μm have been reported^［17］，and effective outputs in the wavelength range of 7‒12 μm have been achieved by LISe-OPA^［18］. However，the relationship between the LISe crystal length and idler energy has not been investigated.

In this study，we use an unannealed LISe crystal grown using the optimized Bridgman method. The optical properties and crystalline quality of LISe are improved by optimizing the growth rate and temperature. The length of LISe crystal is theoretically simulated and optimized，and a 4-mm-long LISe crystal is selected based on the simulation results. We investigate the tuning performance of LISe-OPA in the range from 6.5 to 12 μm and obtained a high energy output at 8.1 μm. The pump-to-idler efficiency and output energy of the idler improved，which is consistent with the simulation results. For high pump energy conditions，an optimal value exists for the length of the NLO crystal to ensure an effective parametric gain and avoid the back-conversion effect. Additionally，the associated spectral and angular acceptances of LISe are investigated.

The experimental setup for the LISe-OPA is described in detail in the literature^［19-20］. A schematic diagram of this device is shown in Fig. 1. The pump source is a mode-locked Nd∶YAG laser（EKSPLA：PL2251A）with a pulse width of 30 ps and a maximum available output energy of 50 mJ at a repetition frequency of 10 Hz. To satisfy the wide tuning requirements of finite aperture NLO crystals，the diameter of the pump beam was scaled down from 10 to 5 mm using a 2∶1 telescope system. The seed source of the LISe-OPA was pulses from a double-pass KTiOPO₄（KTP）-OPG/OPA，which was pumped via the second harmonic of the homologous 1064 nm laser. This stage yielded a continuously tuned NIR output of 1167‒1272 nm as the seeding laser. The energy ratio at 1064 nm was modulated by a half-wave plate（HWP）and a polarization beam splitter（PBS）. Subsequently，it was colinearly mixed with the seed laser through a dichroic mirror after an appropriate time delay and then injected into the LISe crystal. Behind the uncoated LISe crystal，the generated idler laser was separated from the residual pump and the amplified signal using a Ge filter coated with high-transmittance（HT）at the idler laser and high-reflection（HR）1064 nm and 1167‒1272 nm lasers. The idler energy was measured using a thermoelectric energy sensor（OPHIR，PE10-C）. The LWIR spectrum at 10 Hz could not be acquired easily；therefore，we measured the spectrum of the signal. The idler wavelength was calculated based on the law of energy conservation.

In 2017，Wang’s group measured the refractive index of LISe crystal at room temperature using the vertical incidence method in the spectral range of 0.545‒10.928 μm to obtain the Sellmeier equation^［21］ as follows：

where the wavelength λ is expressed in micrometer（μm）. We calculated the type-II phase-matching（PM）angle in the X-Y principal plane of the LISe crystal using 1064 nm laser as the pump source. The corresponding effective NLO coefficient magnitude was calculated by combining the NLO coefficient matrix of the LISe crystal with the effective NLO coefficient expression^［22］，as shown in Fig. 2. Based on the PM result，widely tunable MIR radiation in the 3.2‒‍14 μm range can be achieved theoretically by adjusting the PM angle from 87° to 25° in the X-Y plane for type-II PM interaction with a relatively large effective NLO coefficient（d_eff）of 11.2 pm/V. In the experiment，the LISe crystal was cut at θ=90° and φ=35° to generate the broadest possible MIR-radiation-tuned output.

To select the optimal LISe crystal length，we simulated the ps LISe-OPA process using the three-wave coupling equation^［23］. The pump depletion，linear absorption of LISe，spatial walk-off，group velocity mismatch（GVM），and group velocity dispersion（GVD）were considered in the calculation，where we used parameters corresponding to the device conditions mentioned above. The spatial walk-o‍ff angle of the pump at 1064 nm and that of the idler at 8.1 µm were 18.1 and 16.9 mrad，respectively；and the temporal walk-off（GVM）coe‍fficient between the idler and pump was 403.6 fs/mm. The effects of the spatial and temporal walk-offs were minimal for large spot diameters and short crystals. The idler energy at 8.1 μm vs. the pump energy and crystal length with a spot radius of 2.5 mm for 2-，4-，and 8.3-mm-long LISe crystals is shown in Fig. 3. As shown，the 4 mm LISe enabled the ideal idler energy at a high pump level. The 2-mm-long LISe was not conducive to the efficient generation of idler energy owing to its low parametric gain. Additionally，under an extremely long LISe crystal，the back-conversion effect may be the main contributor to the reduced conversion efficiency.

Considering the parametric gain and damage control of the LISe crystal in the experiment，to obtain a higher idler energy in our experiment，we selected 4 mm as the optimal length，which not only reduced nonlinear absorption but also effectively prevented back conversion under high pump energy levels. Furthermore，short crystals enable a wider range of tuning at the same pass aperture. Therefore，the dimensions of the LISe crystal were determined to be 10 mm × 10 mm × 4 mm，and both of its end faces were optically polished without anti-reflection coating. The transmission spectrum of this 4-mm-long LISe crystal was measured using a Fourier-transform infrared spectrometer，as shown in Fig. 4（a）. The results show that the LISe crystal exhibited relatively high transmittance up to ~9 μm. A decrease in the transmittance beyond 9 μm was similarly observed in previous studies，which was due to the multiphonon absorption of LISe crystal.

As the pump energy increased，the transmittance of the LISe crystal decreased at 1064 nm，as shown in Fig. 4（b）. This was primarily caused by the enhanced pump intensity. As the pump intensity increased，the LISe crystal exhibited nonlinear refraction and nonlinear absorption（including multiphoton absorption and absorption saturation effects）^［24］.

The input-output LISe-OPA characteristics at normal incidence under a seed energy of ~200 μJ are shown in Fig. 5. In all the experiments，the end-face Fresnel losses of the LISe were deducted. At a specified pump energy，the optimal seed energy，which is indicated in the lower right inset，was investigated. The idler energy increased with the seed energy at a fixed pump energy；when the seed energy was ~200 μJ，the generated idler energy saturated. Therefore，in the experiment，we maintained the seed energy at ~200 μJ. As shown in Fig. 5，the idler energy increased monotonically with the pump energy. A maximum idler energy of ~205 μJ was achieved at 8.1 μm under a pump energy of ~19 mJ，which corresponded to a significant improvement of 20 percentage points over previous results^［21］. The maximum photon conversion efficiency was 9.4% at a pump energy of 12.7 mJ. Additionally，at the first point where the conversion efficiency decreased，surface damage was observed at the end face，which corresponded to an on-axis peak intensity of 6.38 GW/cm². The conversion efficiency continued to increase with the pump intensity under a pump energy of less than 12.7 mJ，whereas it decreased slightly when the pump intensity was further increased. The decrease at higher pump levels is primarily attributed to the nonlinear loss of the pump and the back-conversion in the OPA，as well as the point damage of the crystal under a high pump energy.

The tuning performance of the LISe-OPA was investigated. Based on the results of the Sellmeier equation calculations above，conventional birefringent PM for LISe using type-II（e → o + e）1064 nm laser pumping was performed. By tilting the LISe crystal and simultaneously adjusting the corresponding seed wavelength from 1272 to 1167 nm，an idler wavelength range spanning from 6.5 to 12 μm was obtained. The PM curves obtained experimentally agreed well with the theoretical values. The wavelength dependence of the idler energy was measured at a fixed pump energy of ~9.4 mJ，as shown in Fig. 6. At a seed energy of 200 μJ，the idler energy was ~146 μJ at 6.5 μm and ~27 μJ at 12 μm. The idler energy increased as the wavelength decreased owing to the increase in the parametric laser gain，which is inversely proportional to the product of the wavelengths of the signal and idler.

In addition，the angular and spectral acceptabilities of the LISe crystal were measured，as shown in Fig. 7. The angular acceptance obtained by adjusting the incident angle under the normal incidence yielding 8.1 μm was approximately 2.8 mm·deg，and the spectral width acceptance was 24 mm·nm. To eliminate the spectral response of the experimental setup，we performed each set of relative measurements at the same wavelength.

In conclusion，we generated MIR radiation spanning from 6.5 to 12 μm using a high quality LISe crystal with an optimal length of 4 mm. The tuning range was slightly broadened and a higher energy of 8.1 μm was obtained. The idler energy output at a fixed pump energy of ~9.4 mJ was ~146 μJ at 6.5 μm and ~27 μJ at 12 μm. By increasing the pump energy，the idler energy at 8.1 μm increased monotonically，and at the maximum pump energy of ~19 mJ，a maximum idler energy of ~205 μJ was achieved，which was a significant improvement by 20% compared with the previous results. A maximum photon conversion efficiency of 9.4% was achieved at a pump energy of 12.7 mJ. The 4-mm-long LISe crystal investigated in this study showed improved conversion efficiency and the highest idler energy compared with previous results，while enabling easier cutting and processing. Additionally，the angular and spectral width acceptances of the LISe crystal were investigated.