Two-photon imaging of localized optical fields in the vicinity of silver nanowires using a scanning near-field optical microscope

We report a near-field two-photon excitation imaging technique for silver nanostructures to visualize optical fields in the vicinity of nanostructures. We have found that the excitation image of an isolated wire shows an oscillating feature along the long axis of the wire. The period of the oscillation is in good agreement with that of plasmon wavefunctions resonant with the excitation light. The oscillation was thus attributed to the resonant plasmon wavefunction. We also found that bundles of wires exhibit more intense photoluminescence signals than do isolated wires, particularly at junctions between wire ends and the shafts of other wires, which was attributed to the amplified optical fields at the ends of nanowires or interstitial gaps between the wires. We discuss the mechanism of the photoluminescence from silver nanowires. The photoluminescence is most probably attributed to emission from the oxidized surface layer of the silver nanowires, which is enhanced in resonance with plasmons on the wires.