Temporal dynamics of strongly coupled exciton-localized surface plasmons beyond Rabi oscillations

Strong coupling between excitons and light leads to the formation of hybrid states with mixed properties of light and matter. As a result, interesting physical phenomena have been observed at room temperature, e.g. Bose-Einstein condensation and superfluidity, and novel applications are emerging, such as low threshold lasers and quantum devices. Recently it was shown that metasurfaces of aluminum nanoantennas coated with molecular J-aggregates can provide an excellent platform for the formation of strongly coupled exciton-localized surface plasmons (X-LSPs). However, their optical nonlinearities and temporal dynamics are still not well understood. In this work, we use femtosecond pump-probe spectroscopy to study X-LSPs in such composite Al/molecular metasurfaces on time scales that are longer than their Rabi oscillation period. We study the linear and nonlinear optical properties of the uncoupled and hybrid systems and find that the nanoscale plasmonic confinement in metallic nanoparticle cavities introduces intriguing new ultrafast phenomena in the strong coupling regime. These include modifications of the hybrid system due to femtosecond changes in the molecular environment, picosecond oscillations due to acoustic breathing modes of the nanoantennas, and long relaxation times of the nonlinear perturbation at the upper X-LSPs frequency band.