Theoretical and experimental investigation of ultrahigh sensitivity of wavelength-interrogated infrared surface plasmon resonance sensors

An infrared surface plasmon resonance (SPR) sensor with wavelength interrogation was prepared and its refractive-index (RI) sensitivity was theoretically and experimentally investigated. The numerical calculations indicate that at a specific incident angle the sensor has two surface plasmons in the spectral range 700 nm to 2400 nm and that a small change in the clad RI can induce a small red shift of the surface plasmon at a shorter wavelength and a large blue shift of that at a longer wavelength. The experimental results show that at a fixed incident angle the resonance wavelength (λ_R) of the surface plasmon rises linearly with increasing RI in a small RI range and the RI sensitivity rapidly increases from 12024 nm/RIU at λ_R = 846 nm to 57143 nm/RIU at λ_R = 1398 nm. A change of 0.1 wt% in the concentration of aqueous urea solution, corresponding to a RI change of 1.5 × 10^-4, was easily detected with the IR-SPR sensor.