Real-Time Multidimensional Spectroscopy for Organic Molecules in Cavities: Dark-State Dynamics and Optical Signatures

Ultrafast multidimensional spectroscopy has emerged as a powerful tool for probing organic exciton–polaritons in optical microcavities, revealing rich dynamical properties at the femtosecond timescale. However, the dark states and their crosstalk with polaritons still remain challenging, due to the ultrafast dynamics and optical responses that lead to large ambiguities in the interpretation of time-domain spectroscopic experiments, which signify the properties of highly excited states. Here, we develop a fully quantum-mechanical approach to simulate ultrafast two-dimensional polaritonic spectroscopy (2DPS) of organic systems with strong vibronic coupling. By incorporating the time-dependent density matrix renormalization group (TD-DMRG) method into spectroscopic simulations, our approach enables direct visualization of dark-state dynamics and their excited-state absorption (ESA) signals in organic molecules confined in Fabry–Pérot cavities with realistic vibrational environments. Our results reveal two distinct classes of dark states, namely, lower-polariton (LP) polarons and dark polaritons, which are clearly distinguishable from bright polaritons in 2DPS signals. Time-resolved spectra further exhibit their relaxation dynamics, showing by separated timescales and pathways for energy migration, e.g., ∼200 fs for polaron decoupling. Furthermore, these findings provide new theoretical insights into light-induced phases in organic materials and offer guidance for the design of polaritonic devices.