## 3 Results and Discussion
### SEIRAS Substrate Employing Angle-Tunable Absorber
Additionally, this value is comparable to even larger than the enhancement of some reported plasmonic sensors [38, 39]. We further investigate the absorption spectra with different thickness of molecules layer to evaluate the confinement, as shown in Figure S3. The enhanced signal strength increases significantly when the thickness is smaller than 80 nm, whereas the signal strength is hardly increased after the thickness is larger than 80 nm, indicating that the electromagnetic energy is mainly confined within 80 nm from the dielectric surface. The angle-tunable property of the absorber makes it possible to sense the molecular films with two or even more vibration modes while using the same substrate in the wide waveband. We further consider the two chosen individual vibration modes of PTCDA film located at 6.225 \(\upmu\)m and 6.355 \(\upmu\)m  
Figure 5: **(a)** The dielectric permittivity of the molecular film with one vibrational mode at 6.225 \(\upmu\)m when N\({}_{\text{n}}\) = 0.1 mol m\({}^{\text{-3}}\), \(\gamma_{\text{n}}\) = 8 \(\times 10^{11}\) rad/s. **(b)** Absorption spectra of the absorber covered with (solid line) and without (dashed line) 10-nm-thick molecules layer for transverse magnetic (TM)-polarized light with \(\theta\) = 2 deg. **(c)** Absorption spectra of absorber covered with and without molecules in different incident angle. **(d)** Delta absorptance of absorber covered with molecules in different incident angle. to illustrate the frequency-selective enhancement of molecular vibrational modes (labeled as A mode and B mode shown in Figure 6a). The absorptance of the absorber covering the molecules is mapping as a function of wavelength and incident angle in Figure 6b. It is intuitively observed that the interaction between the molecular vibrational modes and the dielectric guide mode near-field leads to a splitting of the infrared absorption spectra at the position of the molecular absorption lines, which significantly enhances the molecular absorption of A and B modes. This is due to the large energy exchange between the dielectric hybrid guide modes and the molecular vibrational modes when they are spectrally matched by tuning the incident angle [40]. To be more specific, the absorption spectra of the absorber covered with the molecules film having two vibrational modes are further extracted from Figure 6b and illustrated in Figure 6c. It can be seen that the resonant peak exhibits a strong dip at the A mode when the incident angle is 2 deg, while it exhibits a strong dip at the B mode with the \(\theta=4\) deg. The delta absorptances of the two vibrational modes are calculated to further quantitatively evaluate the signal enhancement, as shown in Figure 6d. It is obvious that the maximal differential absorption of A mode at 6.225 \(\upmu\)m is 26.8% at the incident angle of 2 deg, thus corresponding to the enhancement of 50-folds. Subsequently, it decreases to 3.03% when the angle increases from 2 deg to 4 deg, which is expected since the guide mode resonant wavelength is tuned away from this mode. However, the signal strength of B mode increases rapidly since the guide mode resonant wavelength is tuned towards B mode at 6.355 \(\upmu\)m. At \(\theta=4\) deg, the guide mode resonance is approximately adjusted to the B mode, giving the large signal  
Figure 6: **(a)** The dielectric permittivity of the molecular film with two vibrational modes at 6.225 \(\upmu\)m and 6.355 \(\upmu\)m when N\({}_{\text{A}}\) = N\({}_{\text{B}}\) = 0.1 mol m\({}^{-3}\), \(\gamma_{\text{A}}\) = \(\gamma_{\text{B}}\) = \(8\times 10^{11}\) rad/s. **(b)** Absorptance map of the dielectric-metal absorber covered with molecules film with different incident angle. **(c)** Absorption spectra of absorber covered with and without molecules film in different incident angle. **(d)** Delta absorptance of absorber covered with molecules film in different incident angle. strength of 26%.