We investigate the transmission properties of Fabry–Pérot microcavities made of metal mirrors containing the organic dye molecule, generally called Lemke. We synthesized Lemke dye and fabricated the microcavities by using simple vacuum evaporation and spin-coating methods. The vacuum Rabi splitting energy increases in proportion to the square root of the absorption coefficient of the cavity layers and exceeds 1 eV at maximum. The ratio of the vacuum Rabi splitting energy to the matter transition energy reaches 0.42, so we consider that the ultrastrong-coupling regime was attained. The dispersion relation is reasonably interpreted by using the full Hopfield Hamiltonian.

We investigate the ultrafast transient transmission properties of an ultrastrongly coupled microcavity made of metal mirrors containing organic dye molecules, generally called a Lemke dye. We perform a dual-color pump-probe spectroscopy on the microcavity and observe photoinduced absorption at the photon energy, corresponding to vacuum Rabi splitting when the lower polariton (LP) branch is pumped. Photoinduced absorption is not observed in the same measurement for the naked Lemke dyes. Therefore, it could reasonably result from the transition from the LP state to the new state that is formed by the interaction between the cavity field and matter, for instance, the upper polariton state. The relaxation time of the photoinduced absorption matched that of bleaching of the LP state. These results indicate that a transition between polariton branches is likely observed.

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