We have simulated a four-oscillator, as well as separate single-oscillator Lorenz models for each of the polyimide peaks interacting with the MIM plasmon mode as a function of the constituent metal disc diameters, as shown in Fig. . The plasmon resonance wavelength of the MIM metasurface has a linear dependence on disk diameter. However, when this plasmon resonance overlaps with molecular absorption, mode splitting becomes apparent. When the permittivity is defined by the single-oscillator Lorenz model (Fig. )), a conventional dispersion curve is obtained. However, when additional bands are included and neighboring Lorenz modes start to partially overlap each other, as in the experiment case of polyimide, the dispersion curve acquires a sigmoidal shape. This is a manifestation of the interaction observed as mode splitting. The normalized coupling strength η between the plasmon resonance and a molecular vibrational mode is described by 2 :