## 1 Introduction  
Surface-enhanced infrared optical sensing technology is powerful for direct label-free analysis with high reliability [1; 2]. This technology can detect minute refractive index (RI) change of analyte [3; 4; 5]; more importantly, it can identify the vibrational mode of target molecules by combining with Fourier Transform Infrared Spectrometer [2; 6], which plays an increasingly important role in chemical detection, food safety, and biosensing [7; 8; 9]. As a kind of fundamental device for surface-enhanced infrared sensing, plasmonic perfect absorbers based on varying metallic structures have been widely employed, such as Metal-Insulator-Metal (MIM) structure [10; 11; 12], Vertical-Square-Split Ring (VSSR) structure [13], Semiconductor-Metal-Semiconductor (SMS) structure [14], Hybrid metasurface [15], and so on. However, the resonance spectra of these absorbers possess full width of half maximum (FWHM) far larger than tens even hundreds of nanometers, owing to the inevitable Ohmic loss of metals, especially when the working wavelength is extended to the important infrared fingerprint region [16; 17; 18]. This results in a low figure of merit (FOM) and significantly hampers the engineering of infrared sensor with high performance.  
Several strategies have been developed to narrow-down the linewidth of resonance spectra, such as adopting gain media to compensate for the losses of metal material [19; 20], designing complex nanostructures for modes hybridization [17; 21], and synthesizing alternative low-loss plasmonic materials [22; 23], to address this obstacle. However, these approaches usually accompany more complex configurations or synthesizing procedures, which greatly limits the practical application of narrowband absorbers in surface-enhanced infrared sensing. To this end, a promising and simple strategy for realizing the narrowband perfect absorbers is to combine guide mode resonance of the dielectric metasurface with the high absorption features of metal [24; 25; 26]. In such a strategy, the free loss dielectric metasurface responses for the generation of narrowband guide mode resonance with high quality factor, while the metal can induce a large modulation depth that is beneficial for detecting the minute variations of surrounding media [19; 27]. These advantages provide a brand-new opportunity for designing the perfect absorber based optical sensors with high FOM. In the visible ranges, by placing the periodical dielectric disk array on metal film, narrowband perfect absorbers have been obtained to detect refractive index change with impressive performance [28; 29]. Whereas, in the important infrared region, narrowband perfect absorbers are still lacking for the systematical investigation, especially in infrared vibrational fingerprint sensing, which is complementary to refractive index sensing and allows for us to achieve more information of the analyte [4; 30; 31].  
Inspired by this idea, here we theoretically present a surface-enhanced infrared sensor employing an absorber based on dielectric-metal metasurface. Such an absorber possesses the features of narrowband and angle-dependent resonant absorption, which enable us to perform refractive index sensing and selectively surface-enhanced infrared absorption spectroscopy in one single substrate simultaneously. Specifically, a narrow resonant absorption peak with high quality factor can be obtained due to the loss-free feature of dielectric and high absorption feature of metal. This feature makes the absorber an outstanding refractive index sensor with high sensitivity and FOM. Additionally, the resonant absorption can be actively tuned by adjusting the incident angle, significantly broadening the working waveband of the absorber. This feature makes the absorber an excellent surface-enhanced infrared absorption spectroscopy (SEIRAS) substrate that can selectively detect multi-vibrational modes of molecules in a wide waveband without any change in the structural parameters.