Two-dimensional femtosecond stimulated Raman spectroscopy for molecular polaritons: Dark states and beyond

We propose a scheme based on femtosecond stimulated Raman spectroscopy (FSRS) for N molecules in an optical cavity. With theoretical simulations, the scheme can access the collective dynamics of molecular polaritons and their coupling to vibrations, along with crosstalk between polariton and dark states. Through multidimensional projections of the FSRS signal, we identify clear signatures of the dark states, e.g., pathways and timescales that used to be invisible with resonant techniques. A microscopic theory is developed for the polaritonic FSRS, which reveals the interplay between polaritonic population and coherence dynamics. The resulting signal makes the dark states visible, thereby providing a technique for probing the dynamics of dark states and their correlation with polariton modes.