Controlling Quantum Pathways in Molecular Vibrational Polaritons

Molecular vibrational polaritons, hybrid quasi-particles formed by molecular vibrations and cavity photons through strong light–matter coupling, have attracted much attention due to their potential to alter photonic properties and to change the course of chemical reactions. The ultrafast dynamics of molecular vibrational polaritons have been studied to advance our understanding of these interesting hybrid quasiparticles. For example, the roles of dark states, hot vibrational mode relaxation in cavities, and manipulation of nonlinear polariton-polariton interactions have been investigated by ultrafast spectroscopy. However, the coherent nature of polaritons makes many viable quantum pathways able to contribute to ultrafast spectroscopic signals, such as two-dimensional infrared spectroscopy (2D IR), complicating its interpretation. Here, using a mid-IR pulse shaper, we report an experimental way to directly measure polariton nonlinear responses involving specific quantum pathways by eliminating others. For example, either coherence or population states may be exclusively probed. This work is useful for the development of polariton-based quantum technologies, and research into the role of coherence in energy transportation in artificial and natural energy materials, and cavity chemistry.