Optimizing Bulk and Surface Sensitivity with Fiber-Tip Leaky Mode Resonances Excited by Low Refractive Index Overlay

Optical surface wave excited by nanocoatings has enabled fiber-optic sensors with high sensitivity to the change in external environment generally characterized by bulk refractive index (RI). Nonetheless, this sensitive response inevitably suffers from the contamination of surface-localized binding events. Herein, we proposed and demonstrated a miniaturized fiber-tip leaky mode resonance ( $ {\rm {L^{e}MR}}$ ) sensor capable of unambiguously discriminating bulk and surface RI changes from a theoretical perspective. The fiber-tip $ {\rm {L^{e}MR}}$ sensor consists of a multi-mode fiber tip coated with $ \mathrm{{TiO_{2}}}$ thin film on which a surface-localized binding layer thinner than $ 20 \, \rm nm$ was set equivalent to the change of surface RI. The operation principle was obtained, with the analyses revealing that several fiber-tip leaky modes can be excited with fiber core mode and flexibly tuned by simply altering the thickness of $ \mathrm{{TiO_{2}}}$ thin film, which hence generates non-polarized fiber-tip $ {\rm {L^{e}MRs}}$ identified by the signature of attenuation maxima in reflection spectrum of the fiber-tip probe. These fiber-tip $ {\rm {L^{e}MRs}}$ present a sensitive linear intensity response (and wavelength shift) to the change in bulk RI (and surface RI), which is considerably feasible for distinctly discriminating bulk and surface RI changes. The results highlight that both the highest bulk sensitivity up to $ 58.5 \, \mathrm{{dB/RIU}}$ in the RI range of $ 1.315 \sim 1.355$ and the highest surface sensitivity up to $ 0.595 \, nm/nm$ , combined with the smallest cross interference between them, can be attained simultaneously with a single fiber-tip $ {\rm {L^{e}MR}}$ and without the need of reference channels, which could enable the development of ultra-compact lab-on-tip optrodes, a pathway of the lab-on-fiber roadmap.