  Two-dimensional (2D) nanomaterials, especially the transition metal sulfide
semiconductors, have drawn great interests due to their potential applications
in viable photonic and optoelectronic devices, such as saturable absorbers
(SAs) and optical switches, etc. In this work, tungsten disulfide (WS2) based
SA for ultrafast photonic applications was demonstrated. WS2 nanosheets were
prepared using liquid-phase exfoliation method and embedded in polyvinyl
alcohol (PVA) thin film for the practical usage. Saturable absorption was
observed in the WS2-PVA SA at the telecommunication waveband near 1550 nm. By
incorporating WS2-PVA SA into a fiber laser cavity, both stable mode locking
operation and Q-switching operation were achieved. In the mode locking
operation, the laser obtained femtosecond output pulse width and high spectral
purity in the radio frequency spectrum. In the Q-switching operation, the laser
had tunable repetition rate and output pulse energy of a few tens of nano
joule. Our findings suggest that few-layer WS2 nanosheets embedded in PVA thin
film are promising nonlinear optical materials for ultrafast photonic
applications as a mode locker or Q-switcher.
