Tuning the Coherent Propagation of Organic Exciton‐Polaritons through Dark State Delocalization

Abstract

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 × 10 6 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.

Unveiling the mixed nature of polaritonic transport: From enhanced diffusion to ballistic motion approaching the speed of light

“…Moreover, a rough estimate of the expansion velocity gives a value of ∼ 176 µm/ps, roughly two thirds the speed of light. This value is more than two orders of magnitude larger than observed for polaritons in any previous time-resolved measurements conducted in Fabry-Pérot cavity systems 7,13,19 . In fact, this estimated velocity approaches the polariton group velocity at the same location on the lower BSWP branch (point B in Fig.…”

“…In particular, we have observed how polaritons, excited in a metallic, planar cavity, gradually migrate and reach distances which are ∼ 5 µm away from their initial position, within several picoseconds. Interestingly, these studies found that the polariton velocity was much lower than expected by simple considerations 7,13 . While the dispersion of cavity polaritons predicts a group velocity on the order of 20 µm/ps, the observed velocities were smaller than 1 µm/ps, pointing toward a fundamental gap in the understanding of cavity polaritons.…”

“…Recently, it has been demonstrated that both excitonic and electronic transport in organic semiconductors can be enhanced by coupling these materials with the photonic modes of an optical resonator [5][6][7][8][9][10][11][12][13] . Under suitable conditions, collective strong coupling between the material excitations and light can give rise to hybrid excitations called polaritons, which are partly excitonic and partly photonic 14 .…”

“…This, in turn, results in quantum correlations between remote molecules 15 , which can facilitate energy transfer [16][17][18] and transport phenomena. This kind of cavity-enhanced transport has been drawing increasing interest over the past few years, with both experimental [5][6][7][8][9][10][11][12][13][19][20][21] and theoretical [22][23][24][25][26][27][28][29] efforts devoted to understanding how strong coupling affects transport phenomena. Although the long-range propagation of polaritons has been directly visualized and studied in steady-state experiments 6,8,10 , constructing a complete picture of their transport dynamics necessitates access to the kinetics of the polaritonic motion, in a similar manner to the various time-resolved 2/22 microscopy techniques used for studying normal organic semiconductors [30][31][32][33][34][35] .…”

“…Although the long-range propagation of polaritons has been directly visualized and studied in steady-state experiments 6,8,10 , constructing a complete picture of their transport dynamics necessitates access to the kinetics of the polaritonic motion, in a similar manner to the various time-resolved 2/22 microscopy techniques used for studying normal organic semiconductors [30][31][32][33][34][35] . Indeed, by employing ultrafast microscopy, a few recent experiments successfully resolved the spatiotemporal evolution of polaritons 7,13 . In particular, we have observed how polaritons, excited in a metallic, planar cavity, gradually migrate and reach distances which are ∼ 5 µm away from their initial position, within several picoseconds.…”