Investigations on Plasmonic Modes of Noble Metal Nano-Disks Using High-Resolution Cathodoluminescence Imaging Spectrosco
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Investigations on Plasmonic Modes of Noble Metal Nano-Disks Using HighResolution Cathodoluminescence Imaging Spectroscopy Anil Kumar1,2, Kin Hung Fung1,3, and Nicholas X. Fang1,3,4 1 Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems at University of Illinois, Urbana-Champaign, IL 61801 USA 2 Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, IL 61801 USA 3 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL 61801 USA 4 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 USA ABSTRACT In this work, we report investigations on plasmonic nano-disks using cathodoluminescence (CL) imaging and spectroscopy. 50 nm thick gold disks fabricated using electron beam lithography were studied and several modes were identified. Detailed analysis of the modes using monochromatic imaging and CL spectra showed strong size dependence. Our investigations on these plasmonic nano-disks allow understanding of light-matter interaction at nanoscale, with several potential applications including next generation plasmonic nano-lasers. INTRODUCTION Surface plasmon polariton excitation using an electron beam offers several unique advantages [1]. Because the incident electrons have wide range of momenta, this approach allows access to large plasmon wave vectors close to the flat region of dispersion curve [1, 2]. Unlike plane wave excitation, electron excitation can be highly localized allowing imaging of dark modes of optical antennas and cavities [3]. Therefore, probing plasmonic nanostructures using electron beam excitation can provide new insights into their underlying physics [3-8], beyond the capability of methods involving optical excitation and imaging, e.g., using Near-field Scanning Optical Microscopy (NSOM). Additionally, investigations using cathodoluminescence (CL) spectroscopy are relatively simpler since no specific sample preparation is required, in contrast to other electron excitation based methodologies, e.g., Electron Energy Loss Spectroscopy (EELS) where the sample needs to be electron transparent [1, 3, 7-8]. Here we report our investigations on plasmonic nano-disks using cathodoluminescence imaging and spectroscopy. These nano-cavities with very small volume [9-13] find several applications including thresholdless laser operation by combining spontaneous emission with the
lasing mode [14]. They are of interest for studying exciton-photon interaction and cavity quantum electrodynamics, and can be potentially used as single photon sources [15]. EXPERIMENT The gold disks were fabricated on specially designed substrates with minimal background luminescence and very low index (Fig. 1). To suppress any background signal, a 100 nm thick gold film was first deposited on Si substrate. Afterwards, 85 nm thick SiO2 layer was deposited using standard PECVD process. 50 nm thick gold nano-disks of various diameters were fabricated using electron-beam lithography with a 3 nm Ti adhesion layer underneath the g
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