Reservoir Dynamics in Vibrational Polaritons Revealed by Detuning-Dependent Ultrafast Spectroscopy

There is an emerging consensus that transient spectral features of cavity vibrational polaritons are dominated by molecular population dynamics of the optically dark reservoir rather than by excited-state transitions within the polariton manifold. In this work, we use ultrafast mid-infrared pump-probe spectroscopy to study vibrational polaritons of ammonium thiocyanate (NH₄SCN) in dimethylformamide (DMF), with Rabi splitting values of 38 cm⁻¹ to 95 cm⁻¹ determined by concentration. The observed polariton line widths show clear robustness to molecular inhomogeneity. By applying second-order kinetic modeling, we extract the dynamics of reservoir states in a zero-detuned cavity from transient polaritonic responses. We find that the reservoir lifetimes are similar to those of the uncoupled molecules. In contrast, under detuned conditions the reservoir relaxes faster regardless of the sign of detuning, suggesting that the cavity modes may open additional relaxation channels. These findings show how cavity properties such as the sample concentration, inhomogeneity, and detunings shape transient polariton spectra.