Effect of Sulfur Alloying on the Femtosecond Nonlinear Optical Response of 3-Monolayer CdSe Nanoplatelets

We report a comparative study of the nonlinear optical (NLO) response of colloidal 3-monolayer (3-ML) CdSe and CdSe0.8S0.2 alloy nanoplatelets (NPLs) in toluene under 800 nm femtosecond excitation. Z-scan measurements reveal that the pure CdSe sample exhibits strong two-photon absorption (2PA) and self-focusing behavior, consistent with high exciton binding energy (EBE) and 1P excitonic transitions. In contrast, the alloyed sample shows reduced nonlinear absorption (NLA) and a self-defocusing refractive response. Open-aperture Z-scan traces for the alloy at low intensities display asymmetries, which we attribute to a sequential absorption process involving 2PA followed by excited-state absorption (ESA) via free carriers. This cascaded mechanism effectively mimics a three-photon absorption (3PA) response. At higher intensities, the solvent contribution dominates, and the asymmetry vanishes. These results demonstrate how sulfur alloying modifies excitonic confinement and carrier dynamics, enabling tunability of NLO behavior in 2D colloidal nanostructures.