Büttner Simon, Philipp Luca Nils, Lüttig Julian, Rödel Maximilian, Hensen Matthias, Pflaum Jens, Mitric Roland, Brixner Tobias
Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan 48109, USA.
J Chem Phys. 2025 Jul 28;163(4). doi: 10.1063/5.0278118.
Coupling molecular transition dipole moments to surface-plasmon polaritons (SPPs) results in the formation of new optical quasiparticles, i.e., plexcitons. Mixing the specific properties of matter excitations and light modes has proven to be an efficient strategy to alter a variety of molecular processes, ranging from chemical reactions to exciton transport. Here, we investigate energy transfer in a plexcitonic system of zinc phthalocyanine molecules aggregated in the crystalline α-phase and an SPP on a planar gold surface. By tuning the angle of incidence, we vary the degree of mixing between excitonic and SPP character of the excited state. We apply our recently developed higher-order pump-probe spectroscopy to separate the system's fifth-order signal describing the dynamics of two-particle interactions. The time it takes for two quasiparticles to meet and annihilate is a measure of their movement and, thus, the transport of excitation energy in the system. We find that the transport extracted from the fifth-order signal is surprisingly unaffected by the mixing ratio of exciton and SPP contributions of the plexciton. Using a rate equation model, we explain this behavior by fast transition from the plexcitonic states to many localized excitonic dark states that do not have an SPP contribution. Our results give an indication of how hybrid exciton-plasmon systems should be designed to exploit the delocalization of the involved plasmon modes for improved transport.
