Observation of ultrastrong-coupling regime in the Fabry–Pérot microcavities made of metal mirrors containing Lemke dye

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.