Ultrafast optical response and ablation mechanisms of molybdenum disulfide under intense femtosecond laser irradiation

Optoelectronics: Digging into molybdenum disulfide Ultrafast damage-causing laser pulses affect bulk molybdenum disulfide differently depending on their energy per area, revealing further insight into the ultrafast electron dynamics. The research group of Lan Jiang from Beijing Institute of Technology, in cooperation with Tianhong Cui of University of Minnesota, has predicted and examined what happened to molybdenum disulfide on electron and lattice level when intense femtosecond laser pulses irradiate. Lower damage-causing energies per area, known as fluences, removed material through inducing a solid-to-gas transition. Higher fluences caused superheating, liquidation, and the formation of nanocracks and nanoridges. Tests on electron dynamics obtained by the materials' reflectivity with different fluences were followed by theoretical simulations, which further helped to reveal the electron and lattice temperature evolution. The findings further understandings for laser processing of molybdenum disulfide and for its use in optoelectronics. Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide (MoS2); however, most of them were confined to the level below the damage threshold. Here the electron dynamics of MoS2 under intense ultrafast laser irradiation was investigated by experiments and simulations. Two kinds of ablation mechanisms were revealed, which led to two distinct types of electron dynamics and final ablation morphology. At a higher fluence, the emergence of superheated liquid induced a dramatic change in the transient reflectivity and micro-honeycomb structures. At a lower fluence, the material was just removed by sublimation, and the ablation structure was relatively flat. X-ray photoelectron spectroscopic (XPS) measurements demonstrated that thermal decomposition only occurred at the higher fluence. Furthermore, a theoretical model was developed to deeply reveal the ultrafast dynamics of MoS2 ablation. The simulation results were in good agreement with the temporal and spatial reflectivity distribution obtained from the experiment. The electron and lattice temperature evolution was also obtained to prove the ablation mechanism. Our results revealed ultrafast dynamics of MoS2 above the damage threshold and are helpful for understanding the interaction mechanism between MoS2 and intense ultrafast lasers, as well as for MoS2 processing applications.