Pulse-width dependent sign reversal of third-order nonlinear refractive index in WS₂ quantum dots

The photo-induced changes in nonlinear refractive index (n₂) of transition metal dichalcogenides (TMDCs) open up the horizon for promising applications in all-optical signal switching. Here, we study the ultrafast third-order nonlinear optical response of WS₂ quantum dots by open and closed aperture Z-scan techniques to get a comprehensive understanding of nonlinear absorption and refraction, respectively. Strikingly, our pulse-width dependent (120 fs and 5 ns) nonlinear spectroscopy measurement reveals a sign reversal of n₂. We demonstrate that free carriers generation obtained by longer (ns) pulse-width excitation is responsible for negative refractive index, showing the relatively large value of 9 × 10^-13cm²/W. On the other hand, the higher concentration of bound carriers (excitons) in shorter (fs) pulse-width excitation is the origin for positive refractive index, resulting in an unusual crossover of n₂. Further, with varying intensity, we observe the transition from saturable absorption to two-photon absorption. Our experimental findings might have provided a new degree of freedom for tuning the third-order nonlinear optical response of TMDCs based materials which can find potential applications in optical modulating and limiting.