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Strong light–matter coupling to form exciton– and vibropolaritons is increasingly touted as a powerful tool to alter the fundamental properties of organic materials. It is proposed that these states and their facile tunability can be used to rewrite molecular potential energy landscapes and redirect photophysical pathways, with applications from catalysis to electronic devices. Crucial to their photophysical properties is the exchange of energy between coherent, bright polaritons and incoherent dark states. One of the most potent tools to explore this interplay is transient absorption/reflectance spectroscopy. Previous studies have revealed unexpectedly long lifetimes of the coherent polariton states, for which there is no theoretical explanation. Applying these transient methods to a series of strong-coupled organic microcavities, we recover similar long-lived spectral effects. Based on transfer-matrix modeling of the transient experiment, we find that virtually the entire photoresponse results from photoexcitation effects other than the generation of polariton states. Our results suggest that the complex optical properties of polaritonic systems make them especially prone to misleading optical signatures and that more challenging high-time-resolution measurements on high-quality microcavities are necessary to uniquely distinguish the coherent polariton dynamics. [ABSTRACT FROM AUTHOR]

While there have been numerous reports of long‐range polariton transport at room‐temperature in organic cavities, the spatiotemporal evolution of the propagation is scarcely reported, particularly in the initial coherent sub‐ps regime, where photon and exciton wavefunctions are inextricably mixed. Hence the detailed process of coherent organic exciton‐polariton transport and, in particular, the role of dark states has remained poorly understood. Here, femtosecond transient absorption microscopy is used to directly image coherent polariton motion in microcavities of varying quality factor. The transport is found to be well‐described by a model of band‐like propagation of an initially Gaussian distribution of exciton‐polaritons in real space. The velocity of the polaritons reaches values of ≈ 0.65 × 106 m s−1, substantially lower than expected from the polariton dispersion. Further, it is found that the velocity is proportional to the quality factor of the microcavity. This unexpected link between the quality‐factor and polariton velocity is suggested to be a result of varying admixing between delocalized dark and polariton states. [ABSTRACT FROM AUTHOR]

Bose-Einstein condensates of exciton-polaritons in inorganic semiconductor microcavities are known to possess strong interparticle interactions attributed to their excitonic component. The interactions play a crucial role in the nonlinear dynamics of such systems and can be witnessed as the energy blueshifts of polariton states. However, the localised nature of Frenkel excitons in strongly coupled organic microcavities precludes interparticle Coulomb exchange-interactions that change mechanisms of the nonlinearity and blueshifts accordingly. In this report, we unravel the origins of blueshifts in organic polariton condensates. We examine the possible contributions: intracavity optical Kerr-effect, gain-induced frequency-pulling, polariton interactions and effects related to saturation of optical transitions for weakly- and strongly-coupled molecules. We conclude that blueshifts in organic polariton condensates arise from the interplay of the saturation effects and intermolecular energy migration. Our model predicts the commonly observed step-like increase of both the emission energy and degree of linear polarization at the polariton condensation threshold. Despite rapid scientific and technological development of organic polariton systems, the understanding of their nonlinear phenomena has eluded the community. This study unravels the mechanisms of the omnipresent energy shifts in organic polariton condensates, underlining the nonlinear dynamics in such systems. [ABSTRACT FROM AUTHOR]

A material system is proposed to generate polariton lasing at room temperature over a broad spectral range. The system developed is based on a boron‐dipyrromethene fluorescent dye (BODIPY‐G1) that is dispersed into a polystyrene matrix and used as the active layer of a strongly coupled microcavity. It is shown that the BODIPY‐G1 exciton polaritons undergo nonlinear emission over a broad range of exciton–cavity mode detuning in the green‐yellow portion of the visible spectrum, with polariton lasing achieved over a spectral range spanning 33 nm. The recorded linewidth of ≈0.1 nm corresponds to a condensate coherence lifetime of ≈1 ps. It is proposed that similar effects can be anticipated using a range of molecular dyes in the BODIPY family; a result that paves the way for tunable polariton devices over the visible and near‐infrared spectral region. [ABSTRACT FROM AUTHOR]