Electron energy-loss spectroscopy on three dimensional plasmonic nanostructures
Final Report Abstract
In this project, we used EELS as a characterization method for the plasmonic near-field of threedimensional metallic nanostructures. We investigated several different plasmonic systems, whose specific properties result from the three-dimensional shape of the structures or depend critically on their thickness. Additionally, we studied in the second subproject planar nanotriangles of different sizes. In the first subproject, we imaged the propagation of plasmon modes on conical metallic nanotips in real space. We observed a pronounced maximum of the EELP for all loss energies near the apex and clear spatial oscillations along the cone, with a decreasing period for higher loss energies. Furthermore, a moderate wavelength compression near the apex was observed. These observations could be attributed to the reflection of the fundamental azimuthal mode of the SPP at the apex, leading to standing-wave interference. In the second subproject, we studied the transition from localized to propagating plasmons in the nearfield. For this purpose, we fabricated gold nanotriangles with sizes ranging between 600 nm and 15 µm by electron beam lithography. In case of the small nanotriangles, we observed in the EELP a number of distinct plasmon modes with well-defined resonance energies. In contrast, the larger nanotriangles show a behavior similar to the propagating surface plasmons on the nanotip. This transition can be understood as follows: The discrete plasmon modes of the two shortest nanotriangles are standing waves that originate from the constructive interference of plasmon waves reflected at both ends of the structure. For the larger nanotriangles, Ohmic losses suppress the feedback to select specific plasmon frequencies and one observes a continuum of plasmon m odes. In the third subproject, we investigated the influence of the metal film thickness on the optical properties of freestanding periodically perforated gold films. For this purpose, we developed a new method to fabricate freestanding nanostructured gold films. In the optical transmission spectra we observed a number of resonances that could be attributed to the excitation of SPPs by the periodic arrangement of hole apertures in the array. With increasing film thickness the reson ances showed a blue shift. In the EELS measurements on the thinnest film (22 nm thickness), the resonances could be attributed to standing waves between the holes. Moreover, we identified an additional dark mode that did not show up in the optical far-field spectra. EELS experiments on the thicker samples were unfortunately not successful (insufficient signal to noise ratio due to the low electron transmission). In the last subproject, we investigated plasmonic hot spots in three-dimensional nanoporous gold films. Nanoporous gold films were fabricated by treating these leaves with nitric acid resulting in the specific dissolution of silver from the alloy. For small electron loss energies, the loss probability featured only small variations across the film while for larger loss energies distinct hot spots could be observed.
Publications
- Real-space imaging of nanotip plasmons using electron energy loss spectroscopy, Phys. Rev. B 92, 085411 (2015)
B. Schröder, T. Weber, S. V. Yalunin, T. Kiel, C. Matyssek, M. Sivis, S. Schäfer, F. von Cube, S. Irsen, K. Busch, C. Ropers, S. Linden
(See online at https://doi.org/10.1103/PhysRevB.92.085411) - Near-field study on the transition from localized to propagating plasmons on 2D nano-triangles, Optics Express 25, 16947-16956 (2017)
T. Weber, T. Kiel, S. Irsen, K. Busch, S. Linden
(See online at https://doi.org/10.1364/OE.25.016947) - Freestanding metasurfaces for optical frequencies, Optics Letters 44, 2105 (2019)
M. Prämassing, T. Leuteritz, H.-J. Schill, A. Fassbender, S. Irsen, S. Linden
(See online at https://doi.org/10.1364/OL.44.002105)
