Theoretical Design of Broadband Tunable Nanograting-Coupled Fiber-Optic Surface Plasmon Resonance For Advanced Sensing

Fiber-optic surface plasmon resonance (FOSPR) technology has drawn considerable attention to the label-free biosensing field. Currently, all FOSPR sensing devices capitalizing on flat plasmonic film are only performed on the fiber sidewalls, and demonstrated in a limited wavelength range, which is unfavorable to broaden their practical sensing application. Here, we develop and theoretically demonstrate an effective alternative for the realization of SPR on a flat gold film achieved on the end facet of optical fiber through the diffracting coupling of nanograting. This enables the consecutively adjustment of operating wavelength over a broad spectral range from visible (650 nm) to near-infrared light (1550 nm) through modulating the nanograting period. The sensing performances of the proposed nanograting-coupled FOSPR sensors with different specified resonance wavelengths are numerically investigated from two aspects of bulk refractive index and surface sensitivities. As a result, the highest bulk refractive index sensitivity of 1163 nm/RIU is obtained at the operating wavelength of 1550 nm and the optimal surface sensitivity is dependent on the location away from top surface of structure. The dependence of incident polarization direction on the sensing performances of the proposed structure are also explored. Due to the ease of adjustment of operating wavelength and compact integration on the end facet, this sensing platform can further extend the utility of FOSPR sensing technology in the practical application fields of disease diagnosis, environment monitoring, etc.