Mapping of Plasmonic Modes in Gold Nanoparticles by Means of Electron Energy Loss Spectroscopy

Vlastimil Křápek Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic Ai Leen Koh Stanford Nano Shared Facilities, Stanford University, Stanford, USA Lukáš Břínek Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic Martin Hrtoň Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic Ondřej Tomanec Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic Faculty of Science, Palacký University, Olomouc, Czech Republic Radek Kalousek Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic Stefan Maier Department of Physics, Imperial College London, London, UK Tomáš Šikola Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic

In metallic nanoparticles, collective oscillations of free electron plasma strongly couple to the electromagnetic field forming the excitations called localized surface plasmons (LSP). A characteristic feature of LSP is a strong enhancement of electromagnetic field within the surrounding dielectric together with its confinement on the subwavelength scale, which can be utilized to control various optical processes in the visible and near infrared spectral region. For example, a plasmonic particle attached to an optical emitter can be used to enhance the absorption of the exciting radiation, amplify or quench the spontaneous emission rate of the emitter, directing the emitted light or as a coupler for a waveguide in integrated optical circuits.

Electron energy loss spectroscopy (EELS) is a method allowing to study both the spatial and spectral distribution of LSP resonances. The electron beam with a narrow kinetic energy distribution passes through or close to the metallic nanoparticle and induces LSP excitations, whose electric field in turn scatters the electrons inelastically. Scanning the sample and recording the energy loss spectrum at each position allows to infer the LSP properties.

In our contribution we present EELS study of gold crescent-shape nanoparticles. These structures exhibit particularly large field enhancement near their sharp features, support two non-degenerate dipolar (i.e., optically active) LSP resonances and are widely tunable. Depending on the volume and shape, we resolved up to four LSP resonances in every metallic particle in the energy range 0.8 – 2.4 eV. The boundary element method calculations helped to identify the character of the resonances and showed that the highest energy feature is a multi-resonance assembly. The two lowest resonances are of importance due to their dipolar character. Remarkably, they are both concentrated near the tips of the crescent, spectrally well resolved and their energies can be tuned between 0.8 – 1.5 eV and 1.2 – 2.0 eV, respectively. As the lower spectral range covers the telecom wavelengths 1.3 and 1.55 µm, we envisage the use of such nanostructures in infrared communication technology.

vlastimil.krapek@ceitec.vutbr.cz