We report on the observation of a strong coupling between a surface plasmon and an exciton. Reflectometry experiments are performed on an organic semiconductor, namely, cyanide dye J aggregates, deposited on a silver film. The dispersion lines present an anticrossing that is the signature of a strong plasmon-exciton coupling. Mixed states are formed in a similar way as microcavities polaritons. The Rabi splitting characteristic of this coupling reaches 180 meV at room temperature. The emission of the low energy plasmon-exciton mixed state has been observed and is largely shifted from the uncoupled emission.

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