To further elucidate the spectral peculiarities of the vibrational mode coupling to the MIM metasurface structure, FDTD simulations were performed assuming a four-level Lorenz model for insulator layer permittivity, summarized in Fig. ; details of the simulation method and parameters are provided in the Methods section. As plotted in Fig. ), polyimide bands numerically synthesised for simulations exhibited linewidth and relative intensity values comparable to the experimental ones. Similarly, by adjusting fitting parameters the FDTD simulations qualitatively well reproduce the experimental absorbance measurement results. However, it must be noted that the experimental absorption peaks exhibit slightly narrower linewidths than numerically calculated ones, likely attributable to quantization of the molecular vibrational modes not accounted for by the classical Lorenz model.