The injection of a beam of free 50 keV electrons into an unstructured gold surface creates a highly localized source of traveling surface plasmons with spectra centered below the surface plasmon resonance frequency. The plasmons were detected by a controlled decoupling into light with a grating at a distance from the excitation point. The dominant contribution to the plasmon generation appears to come from the recombination of d-band holes created by the electron beam excitation.

EPSRC N a noPh otonic s Portfolio Centre, Sc h ool of Ph ysic s a nd A stronom y, U niv ersity of Sou th a m p ton, SO17 1B J , U nited King dom (Dated: Ju ne 2 0 , 2 0 0 5 )The power-flow lines of light interacting with a metallic nanoparticle, in the proximity of its plasmon resonance, form whirlpool-like nanoscale optical vortices. Two diff erent types of vortex have been detected. The ou tward vortex fi rst penetrates the particle near its centerline then, on exiting the particle, the flow-lines tu rn away from the centerline and enter a spiral trajectory. O u tward vortexes are seen for the wavelengths shorter then the plasmon resonance. For the wavelengths longer that the plasmon resonance the vortex is inward: the power-flow lines pass arou nd the sides of the particle before tu rning towards the centerline and entering the particle to begin their spiral trajectory.The structures of optical fi elds around metallic nanoparticles are of special interest due to their role in nanophotonic and plasmonic devices and metawaveguides [1][2][3]. Here for the fi st time, we report that light interacting with an absorbing metallic nanoparticle follows curly trajectories with curvatures on the subwavelength scale, creating whirlpool-like nanoscale optical vortices. These "energy sink" vortices with spiral energy flow line trajectories are seen in the proximity of the nanoparticle's plasmon resonance.O ptical vortices have been identifi ed as features in scalar wavefront dislocations of monochromatic light fi elds and modal lines corresponding to nonmonochromatic light as well as in singularities in the maps representing vectorial properties of light [4]. It is now recogniz ed that singularities are often features of fi elds near sub-wavelength structures. A vortex structure in the streamlines of the Poynting vector has been detected for S ommerfeld's edge diff raction with discussion of the eel-like motion of light at the edge dating back to Newtonian times [5]. Recently vortices were found in light diff racted by narrow slits in silver and silicon [6,7]. However, to the best of our knowledge, vortex fi eld structures have never been detected in the vicinity of metal nanoparticles.We studied the interaction of light with homogeneous isotropic spherical nanoparticles using Mie theory [8] -an exact analytical wave theory giving time-harmonic electromagnetic fi elds E and H at freq uency ω that satisfy the wave eq uationswhere k 2 = ω 2 εµ. S olutions to these eq uations are presented in the form of a series of spherical B essel Functions inside the particle and spherical Hankel functions outside it. The nanoparticle is assumed to have a dielectric coeffi cient ε and permittivity µ. Mie theory gives exact solutions of the vector wave eq uation for the internal and scattered fi elds of the particle and has generated a massive body of literature in which fi eld patterns for angle-dependant scattering, modes of excitation, and integral characteristics such as absorption and scattering cross-section have been calculated [9,1...

We report on the first realization of a hyperspectral imaging technique for surface plasmon polaritons on metallic nanostructures. The technique uses a scanning electron beam and allows for simple visualization of light emission from decoupled plasmons, providing information on decay lengths and feature sizes with nanometer resolution.