The strong coupling between light and matter within optical cavities has opened a new path to modify material properties and chemical reactions. Particularly important for chemical effects are the cases of photonic modes of the cavity resonating with molecular vibrations or electronic transitions, leading to vibrational strong coupling (VSC) and electronic strong coupling (ESC), respectively, where hybrid states of light and matter called polaritons arise. On the other hand, it has been known for decades that the impulsive Raman mechanism can launch molecular vibrations or coherent phonons with A1 symmetry due to a sudden displacement of the equilibrium coordinate upon optical excitation. However, so far, these processes have not been studied in optical cavities with collective effects.

Here, using our recent implementation, which combines numerical propagation of Maxwell’s equations with quantum mechanical simulations of molecules, we have observed the launching of optically-inactive vibrational modes in molecules under ESC inside optical cavities, when driven by an external pulse. The amplitude of the oscillations depends on the energy separation between the two new polaritonic states (Rabi splitting), reaching a maximum when the Rabi splitting matches the molecular vibrational frequency. We explain this mechanism by a periodically driven oscillator that can resonate with the cavity, depending on the number of molecules and mirror conditions. Our results suggest possible spectroscopic ways to detect this novel mechanism with applications in the new field of polaritonic chemistry.