Mechanisms of blueshifts in organic polariton condensates

Abstract

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.

Multimode Organic Polariton Lasing

“…Our approach allows direct calculation of the PL spectrum of the driven system, giving information on the lasing frequency and the evolution of the polariton dispersion, distinguishing photon and polariton lasing. Using our microscopic model, we could show that the blueshift closely matches the occupation of the exciton ground state, corroborating the phenomenological saturation model of polariton interaction [15]. The methods described in this Letter can straightforwardly be extended to more complex molecules (e.g., other electronic states, further vibrational modes, or other dissipative processes), or to analysis of time-dependent pumping which we will discuss in a subsequent publication.…”

“…As seen in many materials [2,3], when pumped sufficiently, such polaritons transition to a "condensed" or lasing state, with macroscopic mode occupation and long range coherence. A wide variety of organic materials have shown polariton lasing [4][5][6][7][8][9][10][11][12][13][14][15] (for a review, see Ref. [16]).…”

Room Temperature Polariton Lasing in Ladder‐Type Oligo(p‐Phenylene)s with Different π‐Conjugation Lengths

“…[ 18,20 ] This blueshift may result from multiple reasons, including repulsive self‐interaction among polaritons, [ 6,19,21,23,35 ] gradual saturation of molecular optical transitions, and intermolecular energy migration. [ 36 ]…”

Cavity Exciton-Polariton Condensates in Engineered Potential Landscapes at Room Temperature

“…Since the increase of the exciton fraction leads to an increment of the polaritonpolariton interactions, one would expect that the blueshift should increase while the cavity is detuned towards higher exciton fraction (higher energy). This lack of correlation with the exciton fraction may be explained by the blueshift being caused by saturation effects [103,104]. Due to the high condensation threshold in MeLPPP compared to semiconductors materials [52] and the low exciton component in this detuning region (∼ 5% excitonic, see Fig 4.3) the saturation of excitons participating in the polariton condensate sits in near the condensation threshold.…”