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John Bigeon2 Nadia Belabas2 Nathalie Bardou2 Christophe Minot2 Alejandro Yacomotti2 Ariel Levenson2 Sylvain Barbay2 Sylvain Le Liepvre2 Simon Vassant1 Fabrice Charra1

2, CNRS, Université Paris-Sud, Université Paris-Saclay, Marcoussis, , France
1, Université Paris-Saclay, CEA Saclay, Gif-sur-Yvette, , France

Hybrid metal/organic subwavelength-structured media can exhibit highly unusual photonic properties. In particular, such media permit a fine tuning of the interactions between constituents, giving birth to new excitation modes, such as plasmon-mediated collective oscillations or strongly-coupled hybrid exciton-polariton waves. This offers original coupling schemes between light and matter of high interest in particular for light emission, light amplification or nonlinear optical processes.
Self-organized molecular systems permit a soft « bottom-up » fabrication of self-replicated structures with hierarchical organizations from the molecular scale up to the macroscopic scale. Liquid crystals embedding various π-conjugated mesogens are outstanding examples combining multi-scale organized structures with fluidity and self-healing. Perylene bisimide dye (PTCDI) is the best archetype as it is amenable to form a large variety of structures and offers a unique combination of (i) one-dimensional nature, (ii) ability for pi-stacking and (iii) intense luminescence even in condensed phases.
We experimentally demonstrated the strong coupling between dense H-aggregate-like PTCDI-based self-organized lamellar phases and surface plasmon polaritons (SPP) propagating at the interface with the gold substrate.[1] Experimental wavevector-resolved reflectance spectra evidence the formation of hybrid states that display a clear anticrossing, attesting the strong coupling regime with a Rabi splitting energy of ~100 meV at room temperature. The strength of the observed strong coupling regime results from the high degree of organization of the dense layers of self-assembled molecules at the nanoscale which results in the concentration of the oscillator strength in a charge-transfer Frenkel exciton, with a dipole moment parallel to the direction of maximum SPP electric field. We compare our results to numerical simulations of a matrix model and reach a good qualitative agreement with the experimental findings.
The effects of exciton-SPP coupling in emission processes was also investigated. Most of the exciton radiative relaxation is funneled into the SPP. At large pumping rates exciton-exciton recombination is the main limiting factor of the optical – or plasmonic – gain in this hybrid architecture.
[1] Strong Coupling between Self-Assembled Molecules and Surface Plasmon Polaritons J. Bigeon, S. Le Liepvre, S. Vassant, N. Belabas, N. Bardou, C. Minot, A. Yacomotti, A. Levenson, F. Charra, S. Barbay, Journal of Physical Chemistry Letters 2017.

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