Comparing Exciton-Plasmon Couplings in J-Aggregate and TMD-Based Hybrids Using Angle-Resolved Two-Dimensional Electronic Spectroscopy.

Recent developments in ultrafast two-dimensional electronic spectroscopy (2DES), a powerful tool to unravel coherent couplings in nanosystems, have enabled studies of strongly coupled collective excitons and surface plasmon polaritons. Here, we report a comparative study of two prototypical systems, plasmonic nanoslit arrays covered with either a J-aggregated molecular thin film or a transition metal dichalcogenide (TMD) monolayer. Using ultrafast angle-resolved 2DES, we probe the coherent polariton response. We show that in both systems dark or weakly coupled excitons play an essential role for the polariton nonlinearity. In J-aggregate-based hybrids, this results in dominant Rabi oscillations with periods differing from the polariton normal mode splitting. In TMD-based hybrids, we observe a largely enhanced polariton nonlinearity and polariton dynamics showing a rapid relaxation into dark states. We rationalize the observations in both systems using phenomenological model Hamiltonian simulations and show that the microscopic origin of the polariton nonlinearities in the two systems are different. Although the characteristic two-exciton blue-shift is dominant in the J-aggregate-based hybrid, many-body interactions result in a pump-induced reduction in normal mode splitting in the TMD-based system. We anticipate that the presented results will be helpful for the design of active exciton-plasmon systems for ultrafast switching and optical information processing.