Fig. 1. (a) Schematic view of the Ag octamer on a Si/SiO2 substrate. The geometry parameters r0, r1, g, and h are set as 100, 75, 15, and 30 nm, respectively. (b) Excitation of the octamer with tightly focused beams. Intensities and transverse polarization distributions of (c) x-polarized Gaussian beam, (d) radially and (e) azimuthally polarized beams at the focal plane. The scale bars in (c)–(e) are 200 nm.

Fig. 2. Normalized (a) scattering and (b) absorption spectra of the octamer under excitation of x-polarized Gaussian beam. Charge distributions for (c) dark and (d) bright modes at the indicated wavelengths in (b). Intensity enhancement maps of (e) dark and (f) bright modes at the top surface of the octamer. The scale bars in (e) and (f) are 100 nm.

Fig. 3. Normalized (a) SHG and (b) intensity enhancement spectra of the octamer illuminated by the x-polarized Gaussian beam. Normalized nonlinear polarization at the top surface of the octamer excited by the fundamental (c) dark and (d) bright modes. The scale bars in (c) and (d) are 100 nm.

Fig. 4. Normalized scattering spectra of the octamer under excitations of (a) radially and (b) azimuthally polarized vector beams. Charge distributions of (c), (d) radial breathing modes (RBMs) and (e) azimuthally arranged dipolar mode (ADM) at the labeled wavelengths in (a) and (b), respectively. The dashed lines in (c) and (d) represent the node positions of these two modes. Intensity enhancement maps of (f), (g) two RBMs and (h) ADM at the top surface of the octamer. The scale bars in (f)–(h) are 100 nm.

Fig. 5. (a), (b) SHG spectra of the octamer normalized by the maximum value of SHG intensity produced by the x-polarized Gaussian beam. (c), (d) Intensity enhancement spectra of the octamer. Here, (a), (c) and (b), (d) are under the excitations of the radially and azimuthally polarized beams, respectively. Normalized nonlinear polarization at the top surface of the octamer excited by the fundamental (e), (f) RBMs and (g) ADM. The scale bars in (e)–(g) are 100 nm.