Femtosecond Soliton Pulse Generation Using Evanescent Field Interaction Through Tungsten Disulfide (WS2) Film

We investigated nonlinear optical characteristics of Tungsten disulfide (WS2) films and experimentally demonstrated their high potential for application as nonlinear saturable absorbers in passively mode-locked fiber lasers. Side polished fiber (SPF) was fabricated and WS2 film was overlaid to provide an efficient evanescent field interaction. The WS2 film was prepared using two methods: liquid phase exfoliation to form few-layer nanosheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS2 on a SiO2 substrate. Two SPF saturable absorbers were prepared by either spin coating WS2 solution or lifting off the multilayer CVD WS2 on SPF. An all-fiber ring cavity was built and the WS2 film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS2 SPF, stable soliton-like pulses were generated with a spectral width of 5.6 nm and 467 fs pulse duration. The fiber laser cavity containing CVD WS2 SPF generated a transform-limited soliton pulse train with a spectral width of 8.23 nm and a pulse duration of 332 fs. Our study confirmed a high potential of WS2 film as a novel 2-D nonlinear optical material for laser applications.