Propagation of high-frequency surface plasmons on gold
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1077-L01-02
Propagation of high-frequency surface plasmons on gold Robert Edwin Peale1, Olena Lopatiuk-Tirpak1, Justin W Cleary1, Samantha Santos1, John Henderson1, David Clark1, Leonid Chernyak1, Thomas Andrew Winningham1, Enrique Del Barco1, Helge Heinrich1, and Walter R Buchwald2 1 Physics, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL, 32816 2 RYHC, Air Force Research Lab, 80 Scott Drive, Hanscom AFB, MA, 01731 ABSTRACT Surface plasmon propagation on gold over 0.1 – 1.6 micrometer distances for plasmon energies in the range 1.6 – 3.5 eV was characterized. Surface plasmons were excited by an electron beam near a grating milled in the gold. The spectra of out-coupled radiation reveal increasingly strong propagation losses as surface plasmon energy increases above 2.8 eV, but little effect in the range 1.6 – 2.8 eV. These results are in partial agreement with theoretical expectations. INTRODUCTION Propagation of electromagnetic signals on metal waveguides via highly-confined, bound electro-magnetic waves known as surface plasmon polaritons (SPP) is central to nano-photonics [1, 2]. Plasmon-electronic integrated circuits (PEIC) have been proposed [3], but the usual optical inputs and outputs for PEIC involve bulky optics. Potentially more compact would be electrical SPP generation and detection, e.g. by electron bombardment using nano-tube field emitters. The spectrum of electron-beam excited SPPs on metals is concentrated at high visible and ultraviolet energies. Although the characteristic propagation length generally decreases with energy for free-electron metals, gold specifically has a propagation length that is expected to remain constant at about 0.3 µm above 2.5 eV (see below). Thus, electron-beam excited, highfrequency SPPs on gold are potentially interesting for nano-scale PEIC applications. Use of a scanning electron microscope (SEM) and cathodoluminescence (CL) to study SPP decay for 110 µm propagation lengths at energies below 2.3 eV was recently described [4-6]. This paper reports an independent experiment of the same type, but where sub-micron propagation is observed at SPP energies above 2.8 eV. EXPERIMENT A nominally 470 nm thick layer of gold was e-beam evaporated onto a 5 nm Cr sticking layer on a polished silicon substrate. This thickness is sufficient that the optical constants are those of bulk gold. Using a 30 keV focused gallium-ion beam (FIB), several 20-line gratings were cut in the gold. The Figure 1 inset presents an FIB micrograph of one of the gratings, where all twenty of the 57 µm-long lines appear in the upper part of the image.
λ (nm): 620
Height (nm)
10
Characteristic length (µm)
20
x
0 -10 -20 -30 -3
-2
-1
0
Distance (µm)
1
2
496
413
354
SPP propagation Field extent above surface
10
1
0.1 2.0
2.5
3.0
3.5
SPP energy (eV)
Figure 1. (left) Atomic force microscopy line scan of grating on gold film. The inset is a focused ion beam micrograph of the grating. The black star schematically represents the ~5 nm diameter electron beam spot
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