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Abstract
In this experimental study, we use a pair of 800 nm 180 fs ultrafast laser pulses to explore time-resolved enhanced ablation of two monolayer transitional metal dichalcogenides (TMDs). A 2D crystal structure gives monolayer TMDs their unique properties including a direct electronic band gap, large excitonic resonances, and strong light-matter interactions. As a result, TMDs are of primary interest in optoelectronics. Our TMD samples are CVD grown molybdenum disulfide (MoS2) and tungsten disulfide (WS2). For comparison, another common ultra-thin material, graphene, is studied. Our optical setup provides sub 100 femtosecond resolution by using a variable delay stage that allows up to 433 ps delay between pulses. Fundamentally, below some fluence threshold, the first incident pulse will interact with the sample, producing a memory that temporarily affects its electronic behavior. If the second pulse arrives before this memory dissipates, an enhanced ablation process will ensue. To characterize the evolution of this phenomenon, we analyze the autocorrelated ablation area-delay and ablation threshold-delay relationships. From our measurements, we extract the lifetimes of the enhanced ablation processes and draw conclusions on the photoelectronic nature of our TMDs. Our results show that the enhanced ablation process in MoS2 has a mean lifetime of ~32 ps with total enhancement ending after 150 ps. This agrees with the partial mean lifetime of C-excitons derived in previous pump-probe experiments. Extracted lifetimes for WS2 demonstrate two possible ablation dynamics. The first lifetime is 58 ps, on par with the relaxation of the A-exciton after excitation, while the second is a much slower decay process in the ~700 ps regime. Graphene shows evidence for coulomb explosion in the sub ps regime with a secondary peak resonance at ~150 ps and no ablation beyond 250 ps. The ablation threshold for MoS2 demonstrates a mostly linear dependence on delay while the WS2 threshold shows two distinct linear regimes. For both, their dependence degrades below ~1ps where bandgap renormalization begins to take place and the carrier dynamics are drastically different from later delays.