Mid-infrared surface plasmon resonance fiber optic sensors.

We propose and experimentally demonstrate what we believe to be the first mid-infrared surface plasmon resonance (SPR) fiber optic sensor using a D-shaped multimode silica optical fiber coated with a 105 nm indium tin oxide (ITO) layer. The sensor shows resonance around 2700 nm, with a refractive index sensitivity of 1065.70 nm per refractive index unit (nm/RIU) for refractive indices ranging from 1.33 to 1.42. Since the evanescent wave outside the fiber decays within approximately 1/3 of its wavelength, the use of mid-infrared wavelengths can significantly increase the probing depth of SPR sensors compared to what is obtained when using visible light. This would allow SPR sensors to probe a larger volume, potentially improving sensitivity and signal-to-noise ratio. In addition, the mid-infrared region aligns with the molecular fingerprint region, which can unlock the potential for gas detection.