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. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement