Z-scan measurement of the nonlinear refractive index of monolayer WS2

Transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS2), are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Recent advances in nanoscale materials characterization and few layer TMDCs' unique optical properties make them a research hot-spot in nonlinear optics. In this work, the nonlinear refractive index of monolayer WS2 has been characterized with Z-scan measurement under 800nm femtosecond pulsed laser excitation, and a value of n(2) similar or equal to (8.1 +/- 0.41) x 10(-13) m(2)/W is obtained. A shift from saturable absorption to reverse saturable absorption was observed at higher input pump intensities in the experiments. The transition process was analyzed using a phenomenological model based on two photon absorption, and the two photon absorption coefficient was estimated about (3.7 +/- 0.28) x 10(-6) m/W. (C)2015 Optical Society of America