Directional Control of Strong Coupling Enables Anisotropic Propagation of Molecular Exciton-Polaritons.

Exciton-plasmon polaritons, leveraging the coherent nature of their photonic component, can overcome the localization of excitations in disordered molecular solids and enable long-range transport. However, a clear understanding of how the optical cavity properties govern polariton transport is lacking. Furthermore, most studies demonstrated unidirectional propagation of polaritons. An active directional control of polariton propagation, crucial for expanding its optoelectronic functionality, remains much less explored. Here, we show enhanced and anisotropic long-range energy transport in molecular aggregates strongly coupled with surface plasmons on two-dimensional plasmonic nanoarrays. The nanoarrays exhibit in-plane angle-dependent, anisotropic, and periodically modulated plasmonic resonant properties (diffraction order, resonance energy and line width) stemming from the anisotropic dispersion of the plasmonic hexagonal lattice, leading to an anisotropic periodic modulation of the cavity-molecule coupling strength. Wavelength-resolved, momentum-selected real-space photoluminescence study reveals enhanced coherent propagation of upper and lower polaritons with ∼10 μm and large group velocities on the order of the speed of light. Crucially, an anisotropic propagation distance contrast of 90% has been achieved by tuning the dispersion and quality factor of plasmonic modes. The results provide valuable guidance for developing coherent, directional energy transport devices for photovoltaics, photocatalysis, optical routing, and organic optoelectronics applications.