Fig. 1. Schematics and simulation results. (a) Schematic of the aggregating process and SERS. (b) Distribution of electric field (E) normalized to incident electric field (E0) and calculated optical gradient force (Fg) exerted on the GNP near a GNF coated nanofiber. (c) Normalized electric field and gradient force for the GNP near a bare nanofiber. (d) Gradient forces as a function of the gap distance (dg) between the GNP and nanofiber with/without a gold nanofilm coated. Inset: enlarged gradient force for GNP near the bare nanofiber. (e), (f) Distributions of normalized electric field for two and three GNPs, respectively.

Fig. 2. Setup and characterization. (a) Schematic of the experimental setup. (b) Scanning electron microscope (SEM) of the nanofiber. (c) Atomic force microscope (AFM) image of the GNF. Inset: height distribution along the white cutline. (d) SEM image of the gold nanoparticles used in experiment.

Fig. 3. Aggregating and releasing of GNPs. (a) Aggregation of GNPs after the laser (785 nm, 10 mW) was turned on (ton=20  s). (b) Aggregation of GNPs at ton=40  s. The detailed aggregation process for ton=20  s to 40 s is shown in Visualization 1. (c) Laser was turned off (toff=0  s). (d) Releasing of aggregated GNPs at toff=40  s. The detailed releasing process for toff=0 to 40 s is shown in Visualization 2.

Fig. 4. SERS of the aggregated GNPs. (a) Raman spectra of R6G molecule solutions with concentrations ranging from 10−12  M to 10−4  M. (b) Raman spectra of R6G molecule solutions with concentrations of 10−10  M and 10−12  M. (c) Intensities of the Raman peak at 1362  cm−1 (I1362) as a function of R6G molecule concentrations. The red line is the linear fit curve of experiment data (square points). The green and blue lines are the main blank signal and blank signal added by a value of 3σ, where σ is the standard deviation of the blank signal. (d) Raman peak positions as a function of R6G molecule concentrations.

Fig. 5. SERS reproducibility of the aggregated GNPs. (a) Raman spectra obtained with a time interval of 1 min with a R6G concentration of 10−6  M. (b) Intensities of Raman peaks. (c)–(f) Distributions of Raman peak positions at 1310, 1362, 1510, and 1648  cm−1, respectively.