Fig. 1. (a) Schematic of electric dipole excitation in Au nanospheres with a backscattering configuration where the excitation is focused onto the sample via a 150× objective and white light is focused through a 20× objective as the side illumination of dark field setup. (b) Dark-field image of Au nanospheres with a radius of 80 nm and a typical zoom-in SEM image, where the scale bar is 100 nm.

Fig. 2. Intensity distributions of (a) linearly polarized Gaussian and (b) radially polarized beams at the wavelength of 633 nm, where the insets indicate the polarization states of the beams. Intensity profiles of the radially polarized beam after passing an analyzer orientated along (c) horizontal, (d) vertical, (e) diagonal, and (f) antidiagonal directions.

Fig. 3. (a) Scattering spectra of individual Au nanospheres excited by a linearly polarized Gaussian beam. From top to bottom, nanospheres have radii of 50 nm, 60 nm, 70 nm, 80 nm, and 90 nm. The left and right columns correspond to the experimental and simulation spectra, respectively. The SEM images of the corresponding Au nanospheres are shown in the middle column, where the scale bar is 100 nm. (b) Scattering spectra of Au nanospheres with different radii calculated from Mie theory, where the crossings represent the resonance wavelengths determined by experimental results in (a), and the inset is the charge distribution of the Au nanosphere (r=80  nm). (c) Electric field enhancement maps of single Au nanospheres with radii from 50 nm to 90 nm (from left to right) at their resonance wavelengths.

Fig. 4. (a) Radially polarized beam-excited scattering spectra of individual Au nanospheres with radii of 50 nm, 60 nm, 70 nm, 80 nm, and 90 nm (from top to bottom). The left and right columns correspond to the experimental and simulation spectra, respectively. The middle column is the SEM images of the corresponding Au nanospheres, where the scale bar is 100 nm. (b) Scattering spectra calculated from Mie theory for Au nanospheres with different radii, where the crossings represent the resonance wavelengths determined by experimental results in (a), and the inset is the charge distribution of the Au nanosphere (r=80  nm) excited by the radially polarized beam. (c) Electric field enhancement maps of single Au nanospheres with radii of 50 nm, 60 nm, 70 nm, 80 nm, and 90 nm (from left to right) at their resonance wavelengths.

Fig. 5. (a) Intensity distribution of the tightly focused radially polarized beam at the xz plane. The green arrows indicate the polarization states around the focal plane (z=0  nm). (b) Evolutions of the scattering spectra of an Au nanosphere (r=80  nm) as its location changes from x=0 to 320 nm, denoted as the dots in (a). The left and right columns correspond to the experimental and simulation results, respectively. (c) Charge (upper panels) and electric field enhancement maps (lower panels) for the Au nanosphere at positions varying from x=0 to 320 nm (from left to right columns). The green arrows denote the orientations of dipole moments. (d) Polarization distributions (green arrows) of the radially polarized beam at its focal plane.