Saturable absorption of few-layer WS and WSe at exciton resonance.

Layered materials such as graphene and transition metal dichalcogenides (TMDs) are excellent candidates as saturable absorbers. However, the saturable absorption of few-layer TMDs is usually measured at the typical wavelengths of lasers, away from the exciton resonances. In this work, we study the saturable absorption effects in one, two, and three-layer WS and WSe, around the A-exciton resonance, using a tunable pulsed laser. Power-dependent reflectance contrast indicates that the saturation power density thresholds are around 5.5 × 10 W/cm (3.8 × 10 W/cm) for monolayer WS (WSe), showing an order of magnitude reduction at the exciton peak. The saturation power density threshold increases with the number of layers. The analytical solution to the dynamics equation illustrates how the laser parameters influence the experimentally measured threshold, highlighting the importance of pulse duration in saturable absorption. This result reveals the effect of exciton decay and exciton-exciton interactions on the nonlinear optical properties of TMDs and benefits the design of TMD-based active optical devices.