Operation characteristics of Mid-infrared optical parametric oscillation pumped by layered WS2 modulated laser
Authors: 王静  , 逄金波  , 郭鹤泽  , 胡新宇  , 周承辰  , 唐文婧  , 蒋锴  , 夏伟  
Published: September 2021

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. At present, the international research on 2D-TMDC-modulated laser is mainly focused on the experimental method, but less on the dynamic theory. In this work, firstly, the characteristics of a homemade 3.5nm-thickness tungsten disulfide (WS2) sample are measured and analyzed. The nonlinear transmission is figured and fitted, which implies the performance of WS2’s saturable absorption. Then, the output characteristics of WS2 saturable absorber (SA) modulated solid-state laser are measured experimentally. Although the photon energy of 1.06 ?m-wavelength laser is less than the bandgap energy of 3.5nm WS2, the sample still exhibits saturable absorption. This may be attributed to the mechanisms of defect-induced absorption, coexistence of states, edge-state of material, and two-photon absorption, etc. Secondly, combined with active acousto-optic (AO) modulator, the active and passive Q-switched OPO with idler-light oscillation is 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 optimization characteristics of WS2 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 WS2 nanosheet could alleviate the "output saturation and drop" phenomenon in singly active-Q-switched OPO. In the end, based on the nonlinear transmittance curve measured for WS2, the absorption cross sections of ground and excited states (1.732×10-17 cm2 and 4.758×10-19 cm2), the lifetime of excited-state energy level (400.6 μs), and the initial population density of ground state (1.741×1022 cm-3) are calculated considering the inhomogeneous-broadening mechanism and non-saturable absorption. The calculated values for the WS2 sample corresponds to the typical value of TMDC SA. The rate equations of OPO modulated by WS2 SA are numerically solved by use of the parameters derived above. The calculated values are in good agreement with the experimental ones. In a conclusion, this work contributes both experimentally and theoretically. In experiment, the optimization effect of 2D TMDC on nonlinear optical conversion is observed, especial the alleviation to thermal effect. In theory, a method to estimate the saturable absorption characteristic parameters of 2D materials is given. It provides reference for the dynamic simulation of 2D materials modulated laser. This work not only experimentally demonstrates the optimization effect of 2D TMDC on nonlinear optical conversion, but also theoretically gives a method to estimate the saturable absorption characteristic parameters of 2D materials. It provides reference for the dynamic simulation of 2D materials modulated laser.