where g 0 is the coupling strength and ω C is the angular frequency of molecular absorption. Based on experimental results, the η values for the weakly coupled modes I to IV (Fig. ) are η I = 6.6%, η II = 4.8%, η III = 3.9%, and η IV = 4.1%, respectively. This result is in good correspondence with FDTD simulations. If the molecular absorption coefficient becomes larger, the η value is likewise increased. Such η values are typical in the MIR wavelength region . However, since η is proportional to √ ω, the values obtained in the MIR spectral region tend to be 2 to 5 times smaller than those reported in the strong coupling case between organic molecules and plasmons at visible wavelengths . Also, the absorption coefficient in polymers at IR is smaller than that of an electron system of dye molecules at visible wavelengths. Most of studies in visible wavelength region were carried out on J-aggregates of polphirine. A molar absorption coefficient of polphirine is ε = 2.6× 10 5 mol/dm -3 cm -1 , while its MIR absorption is in the range of ε = 10 3 -10 4 mol/dm -3 cm -1 . A quantum mechanical description of the molecular vibration mode is :