Cavity molecular dynamics simulations of vibrational polariton-enhanced molecular nonlinear absorption

Recent experiments have observed that the chemical and photophysical properties of molecules can be modified inside an optical Fabry-Perot microcavity under collective vibrational strong coupling (VSC) conditions, and such modification is currently not well understood by theory. In an effort to understand the origin of such cavity-induced phenomena, some recent studies have focused on the effect of the cavity environment on the nonlinear optical response of the molecular subsystem. Here, we use a recently proposed protocol for classical cavity molecular dynamics simulations to numerically investigate the linear and the nonlinear response of liquid carbon dioxide under such VSC conditions following an optical pulse excitation. We find that applying a strong pulse of excitation to the lower hybrid light-matter state, i.e., the lower polariton (LP), can lead to an overall molecular nonlinear absorption that is enhanced by up to two orders of magnitude relative to the excitation outside the cavity. This polariton-enhanced multiphoton absorption also causes an ultrashort LP lifetime (0.2 ps) under strong illumination. Unlike usual polariton relaxation processes-whereby polaritonic energy transfers directly to the manifold of singly excited vibrational dark states-under the present mechanism, the LP transfers energy directly to the manifold of higher vibrationally excited dark states; these highly excited dark states subsequently relax to the manifold of singly excited states with a lifetime of tens of ps. Because the present mechanism is generic in nature, we expect these numerical predictions to be experimentally observed in different molecular systems and in cavities with different volumes.

Automated summary

Recent experiments have shown that the properties of molecules can be altered inside an optical Fabry-Perot microcavity under collective vibrational strong coupling conditions, but this phenomenon is not yet well understood theoretically. By studying the effect of the cavity environment on the nonlinear optical response of the molecular subsystem, researchers have found that a strong excitation pulse to the lower hybrid light-matter state leads to significantly enhanced molecular nonlinear absorption within the cavity. This polariton-enhanced multiphoton absorption also results in an ultrashort polariton lifetime under strong illumination. The mechanism by which energy is transferred to higher vibrationally excited dark states is expected to be observed experimentally in different molecular systems and cavities.