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

In this work, an all-dielectric fiber-tip sensing platform is proposed by which multiple nonpolarized leaky mode resonances (L(e)MRs) are generated for sensitively decoupling bulk and surface characteristics. The key point is to excite fiber-tip leaky modes with a layer of low refractive index (RI) material (here magnesium fluoride thin film) deposited on the fiber-tip end facet, but not high RI films. In this configuration, the fiber-tip thin film works as a planar film waveguide, which is confirmed by the mode equation obtained here. Both the excitation and sensing principles are obtained by analyzing the influence of bulk and surface characteristics on the mode dispersion. Similar with the case of high RI films, the results also verify the capability of decoupling bulk and surface properties with a single fiber-tip (LMR)-M-e excited with low RI film. Another important advance is that both bulk and surface sensitivities can be greatly improved, respectively, up to 107.6 dB/RIU and 16.9 nm/nm (being several tenfolds higher than previous reports). The proposed fiber-tip (LMR)-M-e configuration could open a new paradigm in the domain of "lab-on-optrode" technology, which is highly appealing for multifunctional plug and play biosensing.