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We propose a hybrid metasurface-based perfect absorber which shows the near-unity absorbance and facilities to work as a refractive index sensor. We have used the gold mirror to prevent the transmission and used the amorphous silicon (a-Si) nanodisk arrays on top of the gold mirror which helps to excite the surface plasmon by scattering light through it at the normal incident. We numerically investigated the guiding performance. The proposed absorber is polarization independent and shows a maximum absorption of 99.8% at a 932 nm wavelength in the air medium. Considering the real applications, by varying the environments refractive indices from 1.33 to 1.41, the proposed absorber can maintain absorption at more than 99.7%, with a red shift of the resonant wavelength. Due to impedance matching of the electric and magnetic dipoles, the proposed absorber shows near-unity absorbance over the refractive indices range of 1.33 to 1.41, with a zero-reflectance property at a certain wavelength. This feature could be utilized as a plasmonic sensor in detecting the refractive index of the surrounding medium. The proposed plasmonic sensor shows an average sensitivity of 325 nm/RIU and a maximum sensitivity of 350 nm/RIU over the sensing range of 1.33 to 1.41. The proposed metadevice possesses potential applications in solar photovoltaic and photodetectors, as well as in organic and bio-chemical detection.