Fig. 1.

Absorption and scattering of coupled SPs in a self-assembled NP ring. (A) SEM image of a ring composed of 40-nm gold NPs. The diameter of this ring was 4.6 μm. (B) Scattering spectra of the ring segment (cyan) marked by the cyan rectangle in A and a single NP (orange). The purple, red, and green dashed lines represent the excitation wavelengths of 785, 675, and 514 nm that were used for photothermal heterodyne imaging. The purple, red, and green bars illustrate the spectral bandwidths of 800 ± 20, 700 ± 20, and 550 ± 20 nm that were used for wavelength selective scattering imaging. C, D, and E show the photothermal images of the NP ring with circularly polarized excitation at wavelengths of 785, 675, and 514 nm, respectively. F, G, and H are the unpolarized scattering images of the ring recorded at 800 ± 20, 700 ± 20, and 550 ± 20 nm, respectively. Note that the images are not scaled to each other as they were acquired adjusting the intensity of the incident or detected light so that the signal covered the full dynamic range for each experiment.