## 3 Results and Discussion
### RI Sensor Employing High \(Q\)-Factor Absorber  
The refractive index of the dielectric grating strips (n\({}_{\mathrm{g}}\)) greatly influences the absorption features of the absorber, since the dielectric grating strips are responsible for the generation of the narrow guide mode resonance. As given in Figure 3a, when the refractive index(n\({}_{\mathrm{g}}\)) increases from 1.4 to 2.2, the resonant wavelength redshifts from 6.134 \(\upmu\)m to 8.536 \(\upmu\)m, and the FWHM rapidly increases from 29 nm to 374 nm, leading to the dramatic decrease of the Q-factor from 212 to only 23. Moreover, the absorptance of the absorber decreases from 0.99 to 0.33. These results suggest that the dielectric grating strips with a smaller refractive index (n\({}_{\mathrm{g}}\) = 1.4) is more favorable for the optical sensing with a larger Q-factor and absorptance. Figure 2: (**a**) Absorption and reflection spectra of the absorber. The parameters of the dielectric grating strips are P = 5 \(\upmu\)m, W = 4.3 \(\upmu\)m, H = 1.7 \(\upmu\)m; (**b**) Electric field distribution; (**c**) Magnetic field distribution; and, (**d**) Power loss distribution of the absorber at resonant wavelength of 6.134 \(\upmu\)m.  
The structural parameters of the dielectric grating strips are further optimized to obtain the high Q-factor absorber. Figure 3b illustrates the absorption spectra with varying period P when W = 4.3 \(\upmu\)m, H = 1.7 \(\upmu\)m. One can see that the resonant wavelength exhibits a clear red-shift from 6.134 \(\upmu\)m to 6.88 \(\upmu\)m as P increased from 5.0 \(\upmu\)m to 5.8 \(\upmu\)m. The largest Q-factor of 212 can be achieved for P = 5.0 \(\upmu\)m. The absorption spectra of the absorber with varying width W is plotted in Figure 3c. It shows that the resonant wavelength red shift from 5.86 \(\upmu\)m to 6.134 \(\upmu\)m and the absorption increase from 0.75 to 0.99 with the increasing of W. Additionally, it is found that the higher Q-factor can be achieved for the larger W, which can be attributed to the increased electromagnetic confinement that is induced by the strong coupling between the dielectric strips. As for the height H of the dielectric grating strips, it was  
Figure 3: **(a)** Absorption spectra of the absorber with varying RI (n\({}_{\text{g}}\)) of dielectric grating strips. **(b)** Absorption spectra with varying P when W = 4.3 \(\upmu\)m and H = 1.7\(\upmu\)m. **(c)** Absorption spectra with varying W when P = 5\(\upmu\)m and H = 1.7 \(\upmu\)m. **(d)** Absorption spectra with varying H when P = 5\(\upmu\)m and W = 4.3 \(\upmu\)m. **(e)** Absorption spectra with different refractive index of the surrounding environment (n\({}_{\text{s}}\)). **(f)** Resonant wavelength plotted as a function of refractive index n\({}_{\text{s}}\). The parameters of the dielectric grating strips are P = 5 \(\upmu\)m, W = 4.3 \(\upmu\)m, H = 1.7 \(\upmu\)m, n\({}_{\text{g}}\) = 1.4.  
observed that the absorption spectra of the absorber exhibited a red-shift as H increased from 0.5 \(\upmu\)m to 1.7 \(\upmu\)m, accompanied with an increased resonant absorption, as shown in Figure 3d. Moreover, the Q-factor is larger for the larger height of the grating. These results indicate that the spectral features of absorber can be tuned by the structural parameters of dielectric strips to achieve a high Q-factor value. Additionally, the absorption spectra with resonant wavelength located in lower fingerprint regime are given in Figure S2. For the experimentalists who want to tune the resonance wavelength to another spectral regime, he (she) can first chose a grating with a proper period to locate the resonant wavelength into the interested spectral regime, and then optimize the width and height of the grating to obtain the devices with high absorption and Q-factor. The high Q-factor narrowband absorber is firstly employed as a high-performance refractive index sensor in the infrared region.