## 4 Conclusions  
In summary, we proposed a surface-enhanced infrared sensor employing a narrowband perfect absorber based on dielectric-metal metasurface, where a dielectric grating array is resting on an aluminum plane. The excellent features of the narrowband perfect absorber allow for us to realize both refractive index sensing and SEIRAS on a single substrate in the infrared region. It is found that the high sensitivity of 1800 nm/RIU and FOM of 62 RIU\({}^{-1}\) can be obtained when the absorber as a refractive index sensor due to the high Q-factor of the perfect absorber. More importantly, it is used as a SEIRAS substrate to enhance the molecular vibration mode with the high enhancement of 50-folds when the hybrid guided modes of absorber and the vibrational mode of molecules are spectrally matched. Finally, the working waveband of the absorber is significantly broadened by tuning the incident angle, which provides a feasible way to selectively detect multi vibrational modes of molecules in broad range without any change in the parameters of periodicity, width, height, and refractive index of the grating after the device was processed. Our study shows that the dielectric-metal metasurface perfect absorber has promising potential in the design of surface-enhanced infrared sensors.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/7/2295/s1](http://www.mdpi.com/2076-3417/10/7/2295/s1)]([http://www.mdpi.com/2076-3417/10/7/2295/s1](http://www.mdpi.com/2076-3417/10/7/2295/s1)), Figure S1: The cross-section view of the proposed nanostructure, Figure S2: (a) Absorbance of the metasurface absorber with different height H of the dielectric grating strips when P=3 um and W=2.3 um; (b) Corresponding absorptance and Q-factor with different H of the dielectric grating strips. (c) Absorption spectra of the metasurface absorber with different groove width W when P=3 um and H=0.7 um; (d) Corresponding absorptance and Q-factor with different groove width W. (e) Absorption spectra of the metasurface absorber with different period P when W=0.7 um and H=0.7 um; (f) Corresponding absorptance and Q-factor with different period P, Figure S3: (a) Delta absorptance of absorber covered with molecules layer of different thickness. (b) Enhanced signal strength with different thickness, Table S1: The refractive index sensing performance of the proposed absorber relative to those reported in literatures.  
Formal analysis, G.L.; Writing-original draft, G.L.; Data curation, Z.J.; Investigation, J.N. and P.L.; Software, J.N. and C.G.; Methodology, Z.S. and L.D.; Supervision, Z.J. and W.W.; Writing - review & editing, W.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (No. 61875025, 61675037), Chongqing Research Program of Basic Research and Frontier Technology (cstc2017 cyjBX0048), and Fundamental Research Funds for the Central Universities (2018 CDQYGD0022, cqu2018 CDHB1B03), Chongqing basic science and frontier technology research project(cstc2017 cyjAX0237), and the Visiting Scholar Foundation of Key lab of Optoelectronic Technology & Systems(Chongqing University),Ministry of Education. China.  
The authors declare no conflict of interest.