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

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.

Untargeted effects in organic exciton–polariton transient spectroscopy: A cautionary tale

“…Our model suggests that these features inevitably appear near any optical resonances in the ground state due to the complex optical structure of strong-coupled cavities. In contrast to previous studies, 52,62 we find that within practical pump intensity ranges the shape of these features remains constant, and hence power dependence may not be applied to rule them out, but they may be disentangled through their unique angular responses. Only when measuring with temporal resolution shorter than the very short expected polariton lifetime do we detect signatures that cannot be accounted for in the phenomenological model.…”

“…We perform transient measurements on a set of low-and high-Q microcavities in the strong-coupling regime and observe long-lived, highly structured spectral responses analogous to earlier reports of photoexcited polaritons. 36,[50][51][52] We find that these signatures persist regardless of the choice of excitation wavelength, even in conditions when no polaritons or molecular electronic states should be excited. Instead, we can describe the critical characteristics of these features using a simple model that only considers polariton contraction, thermal expansion, and bulk refractive index changes.…”

“…Accordingly, several subsequent studies using low-Q metallic cavities have reported intriguingly long polariton lifetimes (>1 ps) and alterations to intrinsic photochemical processes from energy transport to singlet fission. 33,36,[51][52][53][54][55][56] Crucially, the understanding of the dynamics in these experiments depends sensitively on the ability to robustly assign the observed spectral features to specific electronic states. In the studies on low-Q cavities, almost the entirety of the photoexcited response was assigned to photoexcited polaritons, with no direct signatures of the dark states reported.…”

“…While clearly identified in vibro-polariton studies and implicated in the blueshifts of organic polariton condensates, 61 it has been considered and explicitly discarded in the transient spectroscopy of exciton-polaritons. 52,62 Recent work by Liu et al on exciton-polaritons suggests that other effects-the modulation of molecular excited-state absorption spectra by the cavity structure, 62 and carrier heating/thermalization dynamics within metal mirrors 63 -should likewise be taken into account.…”

Understanding radiative transitions and relaxation pathways in plexcitons

“…50 Notably, previous studies underscore the importance of dark states, 50,51 although extrapolating the same physics to plasmonic-based hybrids in the visible regime should be done with care. Recent transient absorption studies at visible wavelengths find that two-particle polaritonic states play an important role, 52 while a two-dimensional electronic spectroscopy (2DES) investigation has measured a relaxation pathway from upper to lower branch without the involvement of the molecular dark states. 53 In this work, we explored the ultrafast dynamics of plexcitons and corresponding dissipation mechanisms in detail.…”