Ultrafast Modulation of Exciton–Plasmon Coupling in a Monolayer WS2–Ag Nanodisk Hybrid System

The rapid advances of nanotechnology and nanophotonics bring new approaches for manipulating light–matter interactions at the nanoscale, for example, by integrating plasmonic nanostructures with two-dimensional transition metal dichalcogenides (TMDs) to achieve strong exciton–plasmon interactions for applications in optical switches, sensing, and photovoltaic devices. Such a TMD-plasmonic coupled system provides a highly unexplored territory toward understanding the exciton–plasmon interactions for ultrafast operations. Utilizing transient absorption pump–probe spectroscopy, here we report an ultrafast modulation of the exciton–plasmon coupling in a monolayer WS2–Ag nanodisk hybrid system that displays Fano resonance at the steady-state regime. Specifically, the instant switch-off of the Fano resonance was observed upon the femtosecond pump excitation, characterized by the photoinduced absorption signal at the Fano resonance frequency. The fast recovery of the Fano resonance starts at the sub-100 fs time scale as a result of the energy transfer from excitons of WS2 to plasmons in Ag nanodisks. The slow recovery lasts for several tens of picoseconds, following the carrier relaxations in the subsystems. The ultrafast modulation of the exciton–plasmon coupling in the integrated TMD–plasmonic hybrid system will offer new opportunities for technologically relevant high-speed active plasmonic devices.