Hybrid-state dynamics of dye molecules and surface plasmon polaritons under ultrastrong coupling regime

The achievement of an ultrastrong coupling regime between surface plasmon modes generated by gold conical pits arrays and excitons associated to squaraine dye is demonstrated. Numerical and experimental steady-state reflection measurements demonstrate a remarkable Rabi splitting of 860 meV, to date the largest reported value involving surface plasmon modes. Furthermore, the dynamics of the hybrid states under the ultrastrong coupling regime is investigated by transient absorption spectroscopy. The results show that the upper bands are too short-lived to be detected, while the lower bands have a relatively shorter lifetime with respect to the bleaching recovery of pure squaraine dye. This result contradicts the behaviour of systems in strong coupling regimes, suggesting a different photophysics between strong and ultrastrong coupled systems.