Operation characteristics of mid-infrared optical parametric oscillation pumped by layered WS<sub>2</sub> modulated laser
Authors: Wang Jing , Pang Jin-Bo , Guo He-Ze , Hu Xin-Yu , Zhou Cheng-Chen , Tang Wen-Jing , Jiang Kai , Xia Wei ,   School of Physics and Technology, University of Ji’nan, Ji’nan 250022, China,   Institute for Advanced Interdisciplinary Research (IAIR), University of Ji’nan, Ji’nan 250022, China
Published: January 2022

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.