Absorption in the molecular vibrational manifold occurs when ∂ µ/∂ Q k = 0. Therefore, if the change of molecular dipole moment becomes larger during oscillation, the absorption coefficient becomes enhanced. Polyimide main absorption band is related to C-N or C=O bond vibrations, as discussed above. A heterogeneous bonding generates a larger absorption coefficient than a CH functional group. The largest difference between UV-Vis and MIR absorption is that during UV-Vis absorption in an aggregated dye molecule (e.g., J-aggregate of polphyrine) a dipole extends over the span molecule, which makes the dielectric moment larger. Conversely, vibrational absorption at MIR wavelengths is weak due to harmonic oscillation of atoms which cause negligible interaction between functional groups, since they are independent of each other. Therefore, in a polymer chain, vibrational mode can be described as ε ∼ N × ε mon where ε mon is the monomer molar extinction, N is the number of polymerized monomers. When number N of oscillators are located in a cavity with transition moment M, the splitting strength η is proportional to √ N and M. We can conclude that one can observe splitting due to a high density of material even with a weak absorption.