Plasmon excitations in metallic nanostructures

Abstract

A new hyperspectral imaging technique and apparatus for imaging plasmon excitations and cathodoluminescence in nanostructures with nanoscale resolution have been developed. The apparatus, based on a scanning electron microscope synchronized with a multi-channel spectrum analyzer, allows for collection and detection of optical electron-induced emission from a sample in two configurations (high efficiency and high angular resolution modes) and in the wavelength region from 350 to 1150 nm with 0.8 nm spectral resolution and high quantum efficiency.Using this instrument it was demonstrated that the injection of a beam of free electrons into an unstructured metal surface creates a highly localized nanoscale source of SPPs. It was shown that on a gold surface a 50 keV electron beam of 10 μA current creates a 10 nW source of plasmons with the spectrum spreading from 350 to 1150 nm. The plasmons were detected by controlled decoupling into light with a grating at a distance from the excitation point. The 30 nm delocalization of the plasmon source at the grating was demonstrated and decay lengths of SPPs were measured.The hyperspectral imaging technique was used to study plasmon emission, induced by an electron beam excitation on gold monocrystal decahedronshaped nanoparticles and dimers consisting of such nanoparticles. It was shown that in 100 nm gold decahedron-shaped nanoparticles electron-induced plasmon emission is excited in the spectral range from 350 to 850 nm. The dependence of spatial and spectral structure of dimer plasmon emission on wavelength and separation between the nanoparticles within the dimer was studied. The excitation of hybridized mode on a dimer with a 50 nm gap between the particles was detected at wavelength 600 nm. It was demonstrated that the electromagnetic field structure near a plasmonic nanoparticle forms a vortex. It was shown that the power-flow lines of linear polarized monochromatic light interacting with a metal lambda/20 nanoparticle, in the proximity of its plasmon resonance, form whirlpool-like nanoscale optical vortices (optical whirlpools). Both spherical and spheroidal particles were studied using analytical Mie theory and the Finite Element method. One of two types of vortices, inward or outward, was observed depending on the sign of frequency detuning between the external field and plasmon resonance of the nanoparticle