Study of the Selection Rules of Molecular Polaritonic Transitions by Two-Photon Absorption Spectroscopy

Strong light–matter coupling provides a new way to manipulate the physical and chemical properties of molecules. The study of intrinsic properties of the light–matter hybrid states (polaritonic states) in such coupled systems is of great importance in both molecular and optical sciences. Here, we explore the selection rules of the exciton–polaritons with respect to the symmetry of the coupled molecules. By using transient one-photon absorption (1PA) and two-photon absorption (2PA) spectroscopies, we find that the selection rules for the transition to the lower polaritonic state are different for 1PA and 2PA excitations in strongly coupled rigid and symmetric J-aggregates, whereas they break down for softer flexible molecules with lower symmetry. The 0-delay transient spectra at 2PA can be different from those under the 1PA condition, which reveals the coexistence of different excited states. Thus, the selection rules for the transition to polaritonic states and the excited state manifold are more complex than previously imagined, and such studies should help to deepen the understanding of light–molecule strong coupling.