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 plasmons͒. 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.

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes. [3][4][5]. The enhancement originates from a combination of the electrostatic lightning-rod effect, which is due to the geometric singularity of sharply pointed structures, and localized surface plasmon resonances which depend sensitively on the excitation wavelength. In most studies, the field enhancement factor is either deduced from theoretical calculations or from ensemble averaged measurements. However, recent spatially resolved SERS measurements on single molecules have shown that the field enhancement may vary by many orders of magnitude between different sites of a colloidal metal surface [6,7]. In order to understand which structures render particularly strong enhancements it is necessary to perform measurements on a single particle level. In this Letter, we demonstrate that local SH generation provides a direct means to determine the field enhancement near a nanoscale structure such as a sharp metal tip.SH generation in the context of near-field optical microscopy has been discussed by various authors [8][9][10][11] but the strong dependence on local field enhancement has not been analyzed. Here, we show that the field enhancement effect localizes SH generation to the very end of a metal tip thereby creating a highly confined photon source. We experimentally demonstrate the strong light confinement and investigate the radiation properties of the SH light. We also show that the tip acts as a local probe for fields polarized along the tip axis (longitudinal fields). This unique property can be used for the mapping of field distributions in optical fields as demonstrated recently with trapped ions [12] and single molecules [13,14].

Light emission resulting from two-photon excited gold nanoparticles has been proposed to originate from the radiative decay of surface plasmon resonances. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blueshift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tunable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.

We exploit a plasmon mediated two-step momentum down-conversion scheme to convert low-energy tunneling electrons into propagating photons. Surface plasmon polaritons (SPPs) propagating along an extended gold nanowire are excited on one end by low-energy electron tunneling and are then converted to free-propagating photons at the other end. The separation of excitation and outcoupling proves that tunneling electrons excite gap plasmons that subsequently couple to propagating plasmons. Our work shows that electron tunneling provides a nonoptical, voltage-controlled, and low-energy pathway for launching SPPs in nanostructures, such as plasmonic waveguides.