Highlight Lecture (Symp. D01)
The ability of metallic nanostructures to sustain localized surface plasmons (LSPs) makes them essential elements of future nanophotonic devices. The optical fields associated with these LSPs are concentrated in volumes much smaller than allowed by the diffraction limit and the field distribution and resonance energies depend critically on the size and shape of the nanostructure. Of particular interest are structures that allow optical fields to be steered (waveguides) or concentrated and focused to a certain position. An additional requirement for applications is a broad bandwidth and low radiative losses of such structures. Some of these requirements can be met by metallic plasmonic tapers which have been studied intensively due to the ability of adiabatically coupling the propagating surface plasmon polaritons along their shaft to the nanometer-localized plasmons at their apex. Therefore, they can find applications in the fields of sub-diffraction-limit nanofocusing, ultrafast photoemission, and near-field optical microscopy.
Gold tapers with very smooth surfaces were prepared at Oldenburg University by electrochemical etching in HCl . We investigate the plasmonic modes of three-dimensional single crystalline gold tapers by means of electron energy loss spectroscopy and numerical calculations. We observe discrete higher-order azimuthal plasmonic modes of the gold taper with an opening angle of ~45° with energy dispersions roughly proportional to the inverse local radius. The importance of phase-matching between electron field and radiative taper modes in mesoscopic structure is demonstrated . The broad spectral feature at the apex is due to a rotationally symmetric (zero angular momentum) optical mode. We further systematically study the changes in the dispersion of higher-order plasmonic modes of gold tapers versus the opening angle of the taper, both experimentally and theoretically. Finite-difference time-domain calculations of 3D-tapers with an embedded relativistic electron source  were conducted in order to unravel the experimental results. Further experiments and calculations at different opening angles will be discussed.
NT gratefully acknowledges Alexander von Humboldt Foundation for the research scholarship. Financial support by the European Union project CRONOS (Grant number 280879-2), the Deutsche Forschungsgemeinschaft (SPP1391, DFGLi580/8-1, INST184/107-1) and the Korea Foundation for International Cooperation of Science and Technology (Global Research Laboratory project, K20815000003) is gratefully acknowledged. The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007/2013] under grant agreement no 312483 (ESTEEM2).
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