Fig. 1. The nanohole/disk array-integrated plasmonic fiber sensing probe: geometric structure and reflection spectra. (a) Schematic illustration of the hybrid nanohole/disk array on the end facet of the multimode fiber and its cross-sectional view with marked structural parameters. (b) Theoretically simulated (orange-filled curve) and (c) experimentally measured (blue-filled curve) reflection spectra of the nanohole/disk array-based fiber probe, where two reflection dips are marked by D₁ and D₂.

Fig. 2. The generation mechanism of two resonant dips of the nanohole/disk-integrated fiber end-facet probe. Normalized electric field distributions at (a) dip D₁ of 627 nm and (b) dip D₂ of 719 nm. Corresponding magnetic field distributions at (c) dip D₁ of 627 nm and (d) dip D₂ of 719 nm. Surface charge distributions of (e) dip D₁ and (f) dip D₂, where the schematic of surface charge distribution on the left side clearly shows calculated results on the right side.

Fig. 3. Preparation process of the Au nanohole/disk array on the fiber end facet and the demonstration of fabrication results. (a) The whole fabrication procedure diagram includes two main sections: the fabrication of the large-scale Au nanohole/disk array on the flexible PET substrate and the transfer of the nanohole/disk array from the flexible planar substrate to the fiber end facet. (b) Photograph and (c) scanning electron microscope (SEM) image of the fabricated centimeter-scale Au nanohole/disk array on the flexible PET substrate. The scale bar is 10 mm in (b) and 1 µm in (c). (d) Side- and top-view microscope photographs of the fiber end facet before and after the transferring process. The scale bar is 400 µm. (e) A mobile phone photograph and SEM image of the fabricated fiber probe mounted in a fiber-optic connector. The scale bar is 1 µm. (f) A photograph of the home-built experimental setup for the transferring process. (g) Photographs showing the batch-fabrication process of 15-nanohole/disk array-based fiber probes, each time using a customized fiber bundle fixture. (h) Normalized reflection spectra of 15 fiber end-facet probes in a single preparation.

Fig. 4. Bulk RI sensing properties of the nanohole/disk-integrated fiber end-facet probe. (a) Schematic of the experimental setup. (b) Measured and (c) simulated reflection spectra of the fiber end-facet probe for NaCl solutions with different RIs. (d) Real-time wavelength shift and (e) intensity change of dip D₁ with varying RI of ambient NaCl solution. (f) Measured and (g) simulated wavelength shift (left longitudinal coordinate) and intensity variation (right longitudinal coordinate) of dip D₁ versus different ambient RIs. The error bar in (f) represents uncertainties given by the standard deviations of three repeated measurements for every data point.

Fig. 5. Surface sensitivity determination and specific monitoring of glycoproteins of the nanohole/disk-integrated fiber end-facet probe. (a) Self-assembly flowchart of alternating PAH/PSS bilayers on the prepared plasmonic fiber end-facet probe. (b) Experimentally measured reflection spectra with the PAH/PSS bilayer number ranging from 0 to 35. (c) Simulated (red dots) and measured (blue dots) wavelength shifts of dip D₁ with increasing the number of PAH/PSS bilayers. (d) Schematic illustration of surface functionalization of the plasmonic fiber end-facet probe for the specific determination of glycoproteins Con A. (e) Reflection spectra of different concentrations of Con A. (f) Wavelength shift response of the bio-functionalized fiber end-facet probe as a function of Con A concentrations.