Localized surface plasmons on nanometric gold particles observed with an apertureless scanning near-field optical microscope

Using an apertureless scanning near-field optical microscope, we obtained wavelength dependent images of an isolated gold nanometric particle laid on a quartz substrate. The images show a sharp resonance around λ=488 nm. The far-field surface plasmon resonance of the spherical particles (diameter is 40 nm) in solution is centered at λ=530 nm. Atomic force microscope images give evidence that the particles spread on the substrate. The near-field blueshift of the resonance is thus explained by excitation of a surface plasmon resonance along the minor axis of an oblate spheroid shaped particle.