Real-Space Imaging of Nanotip Plasmons using Electron Energy-Loss Spectroscopy

Benjamin Schröder IV. Physical Institute, Georg-August-Universität Göttingen, Göttingen, Germany Sergey Yalunin IV. Physical Institute, Georg-August-Universität Göttingen, Göttingen, Germany Thorsten Weber Physical Institute, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Electron Microscopy and Analytics, Center of Advanced European Studies and Research, Bonn, Germany Murat Sivis IV. Physical Institute, Georg-August-Universität Göttingen, Göttingen, Germany Felix von Cube Physical Institute, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Electron Microscopy and Analytics, Center of Advanced European Studies and Research, Bonn, Germany Thomas Kiel Department of Physics, Humbold-Universität zu Berlin, Berlin, Germany Christian Matyssek Department of Physics, Humbold-Universität zu Berlin, Berlin, Germany Stephan Irsen Electron Microscopy and Analytics, Center of Advanced European Studies and Research, Bonn, Germany Kurt Busch Department of Physics, Humbold-Universität zu Berlin, Berlin, Germany Claus Ropers IV. Physical Institute, Georg-August-Universität Göttingen, Göttingen, Germany Stefan Linden Physical Institute, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany

Conical metal nanotips can be used to confine electromagnetic energy to subwavelength scales at their apex [1]. This field localization and the associated enhancement enables the study of various nonlinear effects and provides for novel means of imaging [2,3] and spectroscopy [4]. In addition to a direct optical excitation of the apex, propagating surface plasmon polaritons (SPPs) converging in the nanotip can be resonantly generated on metal tapers via grating coupling [5]. A key element of the understanding and optimization of such structures is the dispersive evolution of propagating SPPs on a conical geometry.

In this contribution, we present a study of SPP excitation on a tapered gold structure by using a scanning transmission electron microscope in combination with electron energy loss spectroscopy. This technique allows for a spatially and spectrally resolved mapping of standing wave patterns on the nanotip’s shaft. The recorded data allows for a quantitative SPP dispersion measurement, which clearly evidences the relevance of adiabatic nanofocusing [6] and indicates significant contribution of higher azimuthal modes with increasing radii. We carry out theoretical simulations of the energy loss probability along a straight line parallel to the cone surface using both a numerical Galerkin time-domain approach and a fully retarded semi-analytical modal expansion. Very good agreement with the experimental data is obtained.

References:

[1] D. K. Gramotnev and S. I. Bozhevolnyi: Nanofocusing of electromagnetic radiation, Nature Photonics 8, 13-22 (2013)

[2] D. Sadiq et al.: Adiabatic nanofocusing scattering-type optical nanoscopy of individual gold nanoparticles, Nano Lett. 11, 1609-13 (2011)

[3] C. C. Neacsu et al.: Near-Field Localization in Plasmonic Superfocusing: A Nanoemitter on a Tip, Nano Lett. 10, 592-6 (2010)

[4] J. Steidtner and B. Pettinger: Tip-Enhanced Raman Spectroscopy and Microscopy on Single Dye Molecules with 15 nm Resolution, Phys. Rev. Lett. 2, 236101 (2008)

[5] C. Ropers et al.: Grating-Coupling of Surface Plasmons onto Metallic Tips: A Nanoconfined Light Source, Nano Lett. 9, 2784-8 (2007)

[6] M. I. Stockman: Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides, Phys. Rev. Lett. 93, 137404 (2004)

schroeder@ph4.physik.uni-goettingen.de