## 3 Results and Discussion
### SEIRAS Substrate Employing Angle-Tunable Absorber  
Besides the refractive index sensing, the narrowband perfect absorber with high electromagnetic field confinement can also be used as the surface-enhanced infrared absorption spectroscopy substrate to identify the target molecules, which is complementary to refractive index sensing, since it can detect molecular vibrational modes. It is worth noting that the absorption spectrum of the present absorber can be actively tuned by the incident angle in a wide waveband, as shown in the absorptance map at oblique incidence in Figure 4a. The absorptance spectra with varying incident angle when \(\theta\) = 0 deg, \(\theta\) = 5 deg, and \(\theta\) = 10 deg are further extracted from Figure 4a and plotted in Figure 4b. We can see that the resonance peak splits into two individual peaks and shift towards the opposite directions as the incident angle increases. One of the resonant peak blue shifts to 5.25 \(\upmu\)m and the other one red shift to 6.80 \(\upmu\)m when \(\theta\) = 10 deg, which significantly broadens the working waveband of the absorber for SEIRAS. We then considered the electromagnetic coupling between the guide modes and infrared vibrational modes of molecules to explore the performance of the proposed SEIRAS substrate. The dielectric constant of the molecules can be described as the Drude-Lorentz type dispersion [35; 36]  
\[\varepsilon(\omega)=1+\sum_{n=1}^{N}\frac{e^{2}N_{n}/(\varepsilon_{0}m_{0})}{ \omega_{n}^{2}-\omega^{2}-i\gamma_{n}\omega} \tag{3}\]  
where m\({}_{0}\) is the electron rest mass, \(\omega_{n}\) is the molecular resonant frequency, N\({}_{n}\) is the volume density of molecular dipoles, and \(\gamma_{n}\) is the molecular damping. Here, we start by considering a 10 nm-thick molecular film as the analyte with one single vibrational mode located at 6.225 \(\upmu\)m, corresponding to vibrational mode of aromatic ring that is commonly seen in many organic matter, such as perylene-3,4,9,10-tetracarboxylic diamondride (PTCDA) [37]. Its dielectric constant that is calculated by Equation (3) is plotted in Figure 5a. Figure 5b presents the absorptance spectra of absorber covered with (dashed line) and without (solid line) a molecular film for TM polarization light with \(\theta=2\) deg, respectively. An enlarged (10-fold) absorption spectrum of this molecular vibrational mode is also plotted (blue line). One can see that the direct absorption of the molecule vibrational mode is low (only 0.54 %), which is attributed to the weak interaction between incident infrared light and the molecules. When the molecules are adsorbed on the absorber surface, the weak vibrational modes are significantly enhanced, which feature strong dips over the resonance peaks. Figure 4: (**a**) Absorptance map of the dielectric-metal metasurface absorber with different incident angle. (**b**) The absorptance spectra with varying incident angle when \(\theta=0\) deg, \(\theta=5\) deg and \(\theta=10\) deg. The resonant wavelength of absorber is further dynamically tuned by the incident angle, so that it can overlap with the molecule vibrational mode to achieve the maximum enhancement. Figure 5c shows the absorption spectra of absorber before (dashed lines) and after (solid lines) being adsorbed by the molecules film with varying incident angle. As expected, the enhanced signals vary in strength and line shape, depending on their spectral position relative to the vibrational mode of the molecular film and the dielectric guide mode of the absorber when these two modes are overlapped. Figure 5d stacks the corresponding enhanced vibrational signals (delta absorptance) of molecules film. Obviously, the vibrational signal that is close to the dielectric guide mode resonance peak much larger amplitude than those wing signals. The maximum signal strength of vibrational mode is 26.8% when the incident angle is 2 deg, which is about 50 times with respect to the intrinsic absorption of the molecule film without resonance enhancement.