However, when a strong coupling between the molecular absorber and MIM plasmonic modes is formed, an optical feature called the anti-crossing appears, which manifests as the maximum absorbance at the molecular vibration spectral position. At this anti-crossing, the resonance is suppressed and the spectral shape appears to be split (Fig. ). All four prominent absorption peaks of PI show the most significant peak splitting among the molecular vibrations investigated in this study, indicating the most strong light-matter interaction. On the other hand, even in the case of the PI absorption peaks from 800 to 1000 cm -1 , which have relatively small absorption coefficients, there is a large splitting that is not dissimilar to the central peak. The absorption at those peaks was even smaller than the absorption of the measured peaks of SU-8 and PI. The coupling efficiency defined as ∆E/ω, was analyzed, where ∆E is the maximum peak division and ω is the angular frequency at the absorption peak. The result is shown in Fig. . At first glance, the results seem to be divided into two groups, which are enclosed by dashed ellipses. The group-1 and group-2 seem to follow a linear correlation with molecular absorption, respectively, indicating that PI, OMR, and SU-8 show the same trend independent of molecular species.