Continuum generation from single gold nanostructures through near-field mediated intraband transitions

A broad visible and infrared photoluminescence continuum is detected from surface-plasmon-enhanced transitions in gold nanostructures. We find that the ratio of generated infrared to visible emission is much stronger for gold nanostructures than for smooth gold films. While visible emission is well explained by interband transitions of d-band electrons into the conduction band and subsequent radiative recombination, the strong infrared emission cannot be accounted for by the same mechanism. We propose that the infrared emission is generated by intraband transitions mediated by the strongly confined fields near metal nanostructures (localized surface plasmons). These fields possess wave numbers that are comparable to the wave numbers of electrons in the metal, and the associated field gradients give rise to higher-order multipolar transitions. We compare photoluminescence spectra for single gold spheres, smooth and rough gold films, and sharp gold tips and demonstrate that the infrared signal is only present for surfaces with nanometer-scale roughness.