Fig. 1. (a) SEM image of an array of Au nanochains on a Nb-doped TiO₂ substrate. (b, c) SEM images of nanochains with different chain lengths (b) and gap distances (c), respectively. (d, e) Plots of the L-mode and T-mode surface plasmon resonance peak wavelengths as a function of the chain length (d) and the gap size (e).

Fig. 2. (a) Sketch map of the light illumination for the PEEM measurements. (b) SEM image of an Au nanochain with the designed gap size of 100 nm; PEEM images of the Au nanochain under different irradiation conditions: (c) mercury lamp, (d) mercury lamp and femtosecond laser pulses, and (e) femtosecond laser pulses. The dashed lines indicate the positions of the hot spots. The in-plane wave vector k_// and polarization E_// for the incident laser pulses are indicated in (e).

Fig. 3. Near-field spectra of nanochains with a fixed gap distance of 10 nm but with three different chain lengths for (a) L-mode and (b) T-mode surface plasmons. (c, d) near-field spectra of nanochains with fixed chain length (N=7) but three different gap distances for L-mode and T-mode surface plasmons, respectively. (e, f) Summary of the dependence of both plasmon resonance peak wavelength on the chain length (e) and gap distance (f). The error bars in (e) and (f) are mainly ±10 nm, which is mainly resulted from the wavelength scan step of 10 nm. The origin of such measurement errors can account for the observed difference in peak wavelength between the L-mode and T-mode for a chain length 1 in (e).

Fig. 4. PEEM images of Au nanochains with different gap distances: (a) 30 nm and (b) 100 nm. The outlines of the nanochains are indicated by dotted lines. (c) 3-D surface plot of the photoemission intensity from one nanochain with the gap distance of 30 nm, A SEM image (bottom) and its corresponding 3-D surface plot (above the SEM image) are also presented to help indicate the location of the hotspots. The in-plane wave vector k_// and polarization E_// for the incident laser pulses are indicated in (a).

Fig. 5. FDTD simulations of the (a, c) electric field distribution and (b, d) surface charge distribution for two nanochains with different gap distances: 30 nm (a, b) and 100 nm (c, d). The inset shows a sketch of the light irradiation (the blue and red arrows present the polarization and the wavelength vector of the incident light, respectively).