Fiber-Optic Surface Waveguide Modes Excited by Inter/Intra Mode Transition for Refractometric Sensitivity Enhancement

The generation of surface waveguide modes in telecommunication-grade optical fibers is highly desirable, since it would enable the development of novel all-fiber devices with high performance. Herein, we carry out a comprehensive theoretical investigation on the intrinsic excitation mechanism and sensing performance of fiber-optic surface waveguide modes supported by a tilted fiber grating (TFG) with nanometric-scale gold and TiO
₂
films. The results reveal that both P-and S-polarized surface waveguide modes originate from the mode transition tuned by the Au-TiO
₂
composite, including inter mode transition associated with two adjacent modes and intra mode transition only related to the mode itself. During the mode transition, the power carried by cladding modes is absorbed by the Au-TiO
₂
composite, thereby yielding a strong surface-localized evanescent field that is sensitive to external medium. It is found that for both polarizations the surface waveguide modes induced by the inter/intra mode transition present almost the same refractometric sensitivity, but with the P-polarization (605.6 nm/RIU) being more sensitive than the S-counterpart (339.2 nm/RIU), the state-of-the-art surface plasmon resonance (539.7 nm/RIU), and also the approach enhanced by high-refractive-index film (7.4 nm/RIU). The results attained here represent a breakthrough since they would open new pathways toward single molecular detection.