Time-resolved energy and fluence dependent carrier dynamics in MoS₂ and WS₂

Monolayer transition metal dichalcogenides (TMDs) exhibit interesting optical properties due to the change from indirect band gap to direct band gap when going from bulk to single layer. We investigate the carrier recombination dynamics in CVD grown MoS₂ and WS₂ on sapphire substrates using femtosecond pump-probe spectroscopy. Utilizing a 3.2 eV pump pulse, we excite electrons into the conduction band and tune the probe energy above, below, and on resonance with the exciton level in each sample. We measure differential reflection signals at each probe energy over a range of pump fluences. Depending on the probe energy compared to the exciton level, we observe different carrier dynamics. We observe evidence of band structure renormalization and its dependence on pump fluence. The time dynamics of the differential reflectivity signals allows us to monitor defect mediated relaxation pathways.