Optics

Physics

Absorption (acoustics)

Polariton

Coupling (piping)

Photoexcitation

Femtosecond

Excited state

Atomic physics

Metallurgy

Materials science

Molecular physics

Optoelectronics

Laser

Ultrafast laser spectroscopy

The Journal of Physical Chemistry Letters

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.

Polariton Transitions in Femtosecond Transient Absorption Studies of Ultrastrong Light–Molecule Coupling