The Role of Long-Lived Excitons in the Dynamics of Strongly Coupled Molecular Polaritons

Abstract

The concept of modifying molecular dynamics in strongly coupled exciton-polariton systems is an emerging topic in photonics due to its potential to produce customized chemical systems with tailored photophysical properties. However, before such systems can be realized, it is essential to address the open questions concerning the nature and strength of electronic interactions between exciton-polaritons and localized excited states in chemical system as well as the proper way to measure such interactions. Here, we use transient optical spectroscopy to investigate dynamical interactions between exciton-polaritons, singlet excitons, and triplet excitons in a molecular singlet fission system that is strongly coupled to an optical microcavity.We identify some of the major limitations to modify molecular dynamics in the strong coupling regime. Simultaneous excitation of cavity polaritons and "reservoir" states, defined as dark polaritons and dark excitons (e.g. triplets) from coupled molecules and excitons from uncoupled molecules, always occurs. In addition, slow conversion from reservoir states to cavity polaritons results in minimal changes to the overall population dynamics. Furthermore, we demonstrate how in addition to the usual population dynamics, transient optical measurements on microcavities reveal information pertaining to modification of the exciton-polariton transition energies due to changes in the population of molecular excited states and the exciton-photon coupling conditions.As a consequence of weak interactions between reservoir states and cavity polaritons, judicious design considerations are required to achieve modified chemical dynamics, necessitating the use 2 of molecular systems with long excited-state lifetimes or strong coupling approaches that require a small number of molecules.

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.…”

“…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. Moving beyond structures based on metallic mirrors, 62,63 we find that these optical effects do not require direct photoinduced carrier heating within the mirror and can be similarly prominent in dielectric structures. Moreover, the richer spectrum of dielectric microcavities provides numerous additional signatures of non-specific photoinduced effects.…”

“…The model in Figure 5a assumes 0.1% of the molecules are photoexcited, a typical degree of excitation in transient absorption experiments, and the resulting signal changes are likewise within the range measured by ourselves and others. 36,[50][51][52]62,63 In previous studies, the contribution of such polariton contraction has been ruled out through intensity-dependence measurements, where it is assumed that the Ω ∝ √𝑁 relation should yield progressively smaller Rabi splitting and thus increasingly shifted polariton resonances as the laser power is increased. 52,62 Our calculations reveal that the assumption behind this approach in incorrect, and intensity dependence cannot be used for this determination (Figure 6).…”

“…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.…”

Polariton-assisted manipulation of energy relaxation pathways: donor–acceptor role reversal in a tuneable microcavity

“…It should be noted that, in a certain approximation, Rabi splitting scales as the square root of the ratio between the number of molecules within the mode volume and the mode volume, or equally, with the square root of molecular density. However, a smaller mode volume, first, allows a large coupling strength to be obtained with much fewer molecules, 40 which significantly changes the number of states in the excitonic reservoir and governs the dynamics of relaxation in the system. Second, we would like to mention that the square root dependence of the coupling strength on the concentration arises from the Holstein–Primakoff transformation used to treat the Dicke Hamiltonian, the full solution of which is rather complicated for many-body systems.…”