We report on the origin of energy-shifts in organic polariton condensates. The localised nature of Frenkel excitons in molecular semiconductors precludes interparticle Coulomb exchange interactionsthe latter being the dominant mechanism for blueshifts in inorganic semiconductor microcavities that bear Wannier-Mott excitons. We examine the contribution of optically induced change of the intracavity non-linear refractive index, gain induced frequency-pulling and quenching of the Rabi splitting, as well as the role of polariton-exciton and polariton-polariton scattering in the energy-shift of the polariton mode at condensation threshold in strongly coupled molecular dye microcavities. We conclude that blueshifts in organic polariton condensates arise from the interplay of the saturation of molecular optical transitions and intermolecular energy migration. Our model predicts the commonly observed step-wise increase of both the emission energy and degree of linear polarisation at polariton condensation threshold.

Inorganic cesium lead halide perovskite nanowires, generating laser emission in the broad spectral range at room temperature and low threshold, have become powerful tools for the cutting-edge applications in the optoelectronics and nanophotonics. However, to achieve high-quality nanowires with the outstanding optical properties, it was necessary to employ long-lasting and costly methods of their synthesis, as well as postsynthetic separation and transfer procedures that are not convenient for large-scale production. Here we report a novel approach to fabricate high-quality CsPbBr3 nanolasers obtained by rapid precipitation from dimethyl sulfoxide solution sprayed onto hydrophobic substrates at ambient conditions. The synthesis technique allows producing the well-separated nanowires with a broad size distribution of 2–50 μm in 5–7 min, being the fastest method to the best of our knowledge. The formation of nanowires occurs via ligand-assisted reprecipitation triggered by intermolecular proton transfer from (CH3)2­CHOH to H2O in the presence of a minor amount of water. The XRD patterns confirm an orthorhombic crystal structure of the as-grown CsPbBr3 single nanowires. Scanning electron microscopy images reveal their regular shape and truncated pyramidal end facets, while high-resolution transmission electron microscopy ones demonstrate their single-crystal structure. The lifetime of excitonic emission of the nanowires is found to be 7 ns, when the samples are excited with energy below the lasing threshold, manifesting the low concentration of defect states. The measured nanolasers of different lengths exhibit pronounced stimulated emission above 13 μJ cm–2 excitation threshold with quality factor Q = 1017–6166. Their high performance is assumed to be related to their monocrystalline structure, low concentration of defect states, and improved end facet reflectivity.

GaAs-based microcavities. [2] However, the applications of III-V and II-VI inorganic semiconductors in polaritonics are relatively limited due to the challenging growth techniques required to create wide bandgap semiconductors together with the necessity of using cryogenic temperatures to create Wannier-Mott excitons. [3,4] Organic semiconductors however comprise the broadest class of strongly coupled materials developed to date and include molecular dyes, crystalline organic molecules, oligofluorenes, and conjugated polymers. [5][6][7][8][9][10][11] Owing to the high quantum yield, large dipole moment of optical transitions and high-binding energy of excitons, organic semiconductors have permitted the physics and applications of polaritonics to be explored at room temperature. [12,13] Polariton lasing is one of the most distinctive nonlinear phenomena related to the collective behavior of exciton polaritons. In contrast to conventional photon lasers, a polariton laser does not necessitate the electronic inversion of population, but is instead driven by a stimulated relaxation to a coherent state during the process of condensation to the ground polariton state. This process has allowed polariton lasers to exhibit significantly lower thresholds compared to photon lasers that have been fabricated using the same device configuration. [14,15] Recently, we demonstrated polariton lasing in the yellow part of the spectrum in an organic microcavity containing the molecular dye bromine-substituted boron-dipyrromethene (BODIPY-Br). [5] In the present paper, we explore the incorporation of another molecular dye of the BODIPY family, namely BODIPY-G1 fluorescent dye, into a microcavity and show that by incorporating a wedged cavity-layer configuration, we can achieve strong coupling over a broad range of exciton-photon detuning conditions. Such structures allow us to controllably access different cavity lengths and thus select the energy of the ground polariton state. We then use this approach to provide evidence of polariton lasing over a broad range of wavelengths (>30 nm) utilizing a single material system.