Fig. 1. (a) Sketch map of the Ag-NPs-coated fiber probe; (b) distribution characteristics of the Ag-NPs on the cross section of the fiber probe; (c) dispersion curve of HE11x/y and TE01 with the decrease in the fiber radius in the bare tapered fiber; energy distributions of (d) HE11x/y and (e) TE01 along the Ag film-coated tapered fiber.

Fig. 2. (a)–(c) Sketch map of the Ag-NPs-coated fiber probe excited by three vector modes, HE11x, HE11y, and TE01, respectively, on a cross section of the fiber probe with R=250  nm; (d)–(f) electric intensity distributions of the local surface plasmon modes corresponding to (a)–(c), respectively.

Fig. 3. Optical microscope images of (a) the bare tapered fiber and (b) the Ag-NPs-coated tapered fiber; (c) SEM image of the tip area of the Ag-NPs-coated tapered fiber; (d) partial enlargement of the surface of the Ag-NPs-coated fiber tip; (e) EDS of the Ag-NPs coating of the SERS fiber probe.

Fig. 4. (a) Sketch map of experimental setup for SERS detection using the Ag-NPs-coated fiber tip internally excited via an AVB. Transverse mode intensity distribution of (b) HE11 and (c) TE01. (d)–(g) Polarization distribution examination results of the generated TE01 mode; (h) Raman spectra of MG (10−5  M) molecules absorbed on the surface of the Ag-NPs-coated fiber probe internally excited with HE11 (black curve) and TE01 (red curve) modes, respectively. Integration time is 10 s, and excitation power is 3.5 mW. (i) Raman spectra of MG with different concentrations (10−9  M, 10−10  M, and 10−11  M) adsorbed on the surface of the Au-NPs-coated fiber probe excited via TE01 mode and Raman spectrum of 10−11  M MG solution adsorbed on the surface of Au-NPs-coated fiber probe (pink curve) excited via HE11 mode. The integration time is 10 s, and the excitation power is 3.5 mW.

Fig. 5. (a) Raman spectra of the MG solution (10−5  M) absorbed on the surface of the silver-coated fiber tip and internally excited via HE11 mode (black curve) and TE01 mode (red curve) after the probe being stored in a lab environment for 3 h. The integration time is 10 s. The excitation power is 3.5 mW. (b) Raman spectra as a function of storage time, recorded at 10 min intervals for 60 min, for the sample stored in a lab environment for 3 h before measuring the Raman spectra under TE01 mode excitation. The integration time is 5 s. The excitation power is 3.5 mW. (c) Time variation of intensity of the 1613.5  cm−1 band as shown in (b).

Fig. 6. (a)–(c) Raman spectra of MG (10−5  M) detected with three different Au-NPs-coated fiber probes and internally excited via HE11 and TE01 modes, respectively. The integration time is 10 s. The excitation power is 3.5 mW. (d) Histogram of the magnification calculated with the intensity of the 1613.5  cm−1 band of the three SERS fiber probes excited using TE01 and HE11 modes.