A Theoretical Model for Linear and Nonlinear Spectroscopy of Plexcitons.

We present a theoretical model to investigate the dynamics and spectroscopic properties of a plexciton system consisting of a molecular exciton coupled to a single short-lived plasmonic mode. The exciton is described as a two-level system (TLS), while the plasmonic mode is treated as a dissipative harmonic oscillator. The hierarchical equations of motion method is employed to simulate energy transfer dynamics, absorption spectra, and two-dimensional electronic spectra (2DES) of the system across a range of coupling strengths. It is shown that increasing the exciton-plasmon coupling strength drives a transition in the absorption spectra from an asymmetric Fano line shape to a Rabi splitting pattern, while coupling the TLS to intramolecular vibrational modes reduces the central dip of the absorption spectra and makes the line shape more symmetric. The simulated 2DES exhibit distinct features compared to those of a coupled molecular dimer, highlighting the unique nonlinear response of plexciton systems. In addition, a "breathing mode" pattern observed in the strong coupling regime can serve as a direct evidence of Rabi oscillation.