  We show that for the optimized angle of incidence, the SPR based optical
sensors exhibit dual resonance in the near-infrared region around which the
sensor becomes exceptionally high sensitive. Both the resonances show opposite
spectral shift with an ambient refractive index, increasing the differential
shift by many folds. The physical reason behind the dual resonance and opposite
spectral shifts are also explained using the modal analysis and the
phase-matching condition. The presence of dual resonance is highly susceptible
to the angle of incidence, which facilitates the sensors to work in a wide
range from gaseous specimen to biological ones with extremely high sensitivity
of 460 um/RIU and 290 um/RIU, respectively. The considered sensor has broader
prospects for the detection of bio-chemicals and gases without changing its
geometrical parameters.
