- The implications of adding thin dielectric film with high refractive index (RI) on top of the transducer metallic layer in surface Plasmon resonance (SPR) sensors are extensively investigated particularly in the infrared in the spectral mode. An over-layer made of silicon with thickness of about 10 nm is introduced on top of the silver metallic layer in SPR sensors when the Kretschmann-Raether (KR) configuration is used. The spectral interrogation is considered and the thickness of the dielectric over-layer is below the cutoff thickness of the lowest guided TM mode. The additional film causes to red-shift the resonance wavelength, narrowing of the dip in the reflectance spectrum, and enhancing the spectral sensitivity and the figure of merit (FOM) of the sensor. Furthermore, adding the top dielectric film enhances the electromagnetic fields at the analyte interface, increases the penetration depth inside the sensed medium, and improves the stability of the sensor.