Abstract

Optical parametric oscillator (OPO) is an important mid-infrared coherent light source. Two-dimensional (2D) transition metal dichalcogenide (TMDC) with nonlinear absorption of near-infrared-wavelength light is expected to be a prospective modulating switch for OPO’s fundamental laser. In this work, firstly, the characteristics of a home-made 3.5nm-thick tungsten disulfide (WS<sub>2</sub>) sample are measured and analyzed. The nonlinear transmission is figured and fitted, revealing the performance of WS<sub>2</sub>’s saturable absorption. Then, the output characteristics of WS<sub>2</sub> saturable absorber (SA) modulated solid-state laser are measured experimentally. Although the photon energy of 1.06 μm-wavelength laser is less than the bandgap energy of 3.5nm WS<sub>2</sub>, the sample still exhibits the saturable absorption. This may be attributed to the mechanisms of defect-induced absorption, coexistence of states, edge-state of material, two-photon absorption, etc. Secondly, combined with active acousto-optic (AO) modulator, the active and passive <i>Q</i>-switched OPO with idler-light oscillation are implemented, and the nanometer pulse-width mid-infrared pulse is obtained. The implementation of AO modulator is to manage the regular switching time to reduce the pulse peak-to-peak vibration of fundamental light and improve the peak power. The optimal characteristics of WS<sub>2</sub> for OPO are studied. Based on the saturable absorption characteristics, the output pulse is compressed by 60%, the peak power is improved by 191%, and the stability of pulse train is improved by 79.62%. Especially, the insertion of WS<sub>2</sub> nanosheet could alleviate the “output saturation and drop” phenomenon in singly active-<i>Q</i>-switched OPO. This phenomenon may origin from the uneven refrigeration of KTA. Because the saturable absorption effect of WS<sub>2</sub> can significantly reduce the transverse area of Gaussian beam, it can alleviate the temperature gradient distribution of KTA and optimize the output characteristics. Finally, based on the nonlinear transmittance curve measured for WS<sub>2</sub>, the absorption cross section of ground state and excited state are calculated to be1.732 × 10<sup>–17</sup> cm<sup>2</sup> and 4.758 × 10<sup>–19</sup> cm<sup>2</sup>, respectively, and the lifetime of excited-state energy level and the initial population density of ground state are evaluated to be 400.6 μs and 1.741 × 10<sup>22</sup> cm<sup>–3</sup>, respectively, by considering the inhomogeneous-broadening mechanism and unsaturated absorption under large signal. The rate equations of layered-WS<sub>2</sub> modulated optical parametric oscillator are solved. This study shows the optimization effect of 2D TMDC on nonlinear conversion of laser, especially the mitigation of thermal effect. At the same time, it provides a parameter basis for the dynamic simulation of two-dimensional material modulated laser.

Highlights

  • This study shows the optimization effect of 2D transition metal dichalcogenide (TMDC) on nonlinear conversion of laser, especially the mitigation of thermal effect

  • 2) (Institute for Advanced Interdisciplinary Research (IAIR), University of Ji’nan, Ji’nan 250022, China)

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Summary

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了约 5 cm–1 的红移, 验证了样品的层状结构 [23−25]. 图 1(b) 测量了 A1g 模式强度的拉曼映射图像, 其 强度的微小变化表明合成 WS2 的高度均匀性 [26]. 在图 1(c) 中, 使用 CX23 型光学显微镜 (Olympus, 日本) 测量了合成样品的光学显微照片, 其颜色对 比度表明了 WS2 的少数层结构 [27]. 图 1(d) 给出 了 WS2 的原子力显微镜 (AFM, Bruker) 的观察结 果. 考虑到单层 WS2 的厚度约为 0.8 nm[28], 从高 度剖面分析的 3.5 nm 厚度表明, 材料具有约 4 层 的二维纳米结构, 与上述拉曼光谱和光学显微照片 的分析结果一致. 在 1.4—3.0 W 的泵浦功率下, 实验记录了脉 冲激光的输出特性, 如图 3(b)—(e) 所示. 图 3(b) 显示了平均输出功率, 激光运转的阈值为 1.4 W, 实验中获得的最高平均功率为 650 mW; 图 3(c) 是 WS2 SA 调制的脉冲宽度, 实验中的最小脉宽约 为 684 ns; 输出激光的脉冲重复率如图 3(d) 所示, 它随着输入能量的升高而升高, 在抽运功率 3 W 下, 得到实验中最大的脉冲重复率约为 259 kHz; 在图 3(e) 中, 计算了脉冲的峰值功率, 在 3 W 抽 运功率下, 脉冲的最大峰值功率约为 3.7 W, 此时 单脉冲能量为 2.51 μJ

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