To confirm the effect that coupling strength has on spectral signatures, a comparison between spectra was carried out with D = 1500 nm discs, outlined in Fig. . It should be noted that at least eight peaks in the absorption spectra of the vibrational modes of polyimide can be identified when Lorenzian curve fitting is applied. According to the rules associated with strong coupling, the interaction between a single plasmon resonance and one molecular absorption peak generates two spectral signatures. Furthermore, the MIM plasmon resonance has a sufficient spectral width to encompass all four vibrational peaks, hence, induces splitting of all four modes into a total of eight signatures (Fig. ). The plasmon resonance overlaps most strongly with peaks II and III. In that case, split modes can be identified as having both upper and lower energy level branches. However, peaks I and IV, which represent a weaker overlap between the MIM plasmon resonance and molecular vibrational modes do not exhibit splitting behavior. If no interaction between the split modes is assumed, the modes in the four-oscillator Lorenz model are expected to appear at the same energy as described by the single-level Lorenz model. However, only the two peaks at the positions I and IV (Fig. ), distinct from the crowded family of peaks mostly derived from II and III modes, match the positions predicted by their single-oscillator Lorenzian models. The other six closely spaced peaks formed from the original four-level Lorenz model have to be analysed in greater detail. Their complexity is consistent with the prominence of the interaction and mode mixing between the neighbouring split modes. In the case of strong coupling, all the peaks still have quality factors The linewidth predicted by FDTD model was found broadened in the experiments. We used the four-polarization method to check for a presence of molecular orientation, which could explain narrowing of the bands . Spectral maps at selected absorption bands showed that there was no molecular orientation in the MIM arrays with evaporated nano-thin polyimide (Fig. ). Interestingly, similar tendencies can be observed even using a spin coated amorphous polyimide film as shown in Fig. . Therefore, it could be concluded that a limitation of the transition probability of the quantized IR absorption levels is the most probable cause of the observed narrowing of the peaks when the molecular vibrations are strongly coupled to the collective plasmonic band. Also, it is expected that a higher coupling strength could be attained using oriented polyimide chain films. The resonance peaks also become shifted from their initial molecular absorption spectral positions. However, in the weak coupling regime, represented by mode I, the line width remains almost unchanged from an uncoupled molecular absorption peak. Therefore, the weak coupling condition with strong absorption enhancement has considerable potential for applications in molecular sensing where narrowband MIR emitter sources are in considerable demand. In this regard, how the strong coupling and mode splitting regime can be gainfully utilized as a narrowband source is less apparent.