Polariton Transitions in Femtosecond Transient Absorption Studies of Ultrastrong Light–Molecule Coupling
Courtney A. DelPo, Bryan Kudisch, Kyu Hyung Park, Saeed‐Uz‐Zaman Khan, Francesca Fassioli, Daniele Fausti, Barry P. Rand, Gregory D. Scholes
Abstract
Strong light-matter coupling is emerging as a fascinating way to tune optical properties and modify the photophysics of molecular systems. In this work, we studied a molecular chromophore under strong coupling with the optical mode of a Fabry-Perot cavity resonant to the first electronic absorption band. Using femtosecond pump-probe spectroscopy, we investigated the transient response of the cavity-coupled molecules upon photoexcitation resonant to the upper and lower polaritons. We identified an excited state absorption from upper and lower polaritons to a state at the energy of the second cavity mode. Quantum mechanical calculations of the many-molecule energy structure of cavity polaritons suggest assignment of this state as a two-particle polaritonic state with optically allowed transitions from the upper and lower polaritons. We provide new physical insight into the role of two-particle polaritonic states in explaining transient signatures in hybrid light-matter coupling systems consistent with analogous many-body systems.