Plasmonic Oscillations in Au Nano-rods Fabricated by Electron Beam Lithography
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Plasmonic Oscillations in Au Nano-rods Fabricated by Electron Beam Lithography Urcan Guler and Rasit Turan Department of Physics, Middle East Technical University, Inonu Boulevard, Cankaya, Ankara 06531, Turkey. ABSTRACT Localized Surface Plasmon Resonances (LSPR) in rod-shaped Gold (Au) nanoparticles patterned with Electron Beam Lithography (EBL) technique are observed via reflectance measurements. Resonance peaks corresponding to the principal axes of the nano-rods are shown to be affected by each other. Excitation of one of the peaks is found to result in a decrease in the peak intensity of the resonance through the other axis. Arrays of Au nanoparticles with constant width and thickness but increasing length are examined for further understanding of the effect. As the particle length increased from 70 nm to 300 nm, resonance peak wavelength shifted from 650 nm to 1200 nm. Total reflectance intensities of samples with varying principal axis dimensions obtained through the spectral region of interest are also examined to see the relation between contributing electrons and total amount of reflected intensity. Results corresponding to both polarized and unpolarized illumination of samples are presented together to gain better understanding of lowered reflectance peak intensities obtained from the latter case. Based on the results obtained so far, nano-sized metal rods are promising tools for optically switched intensity modulation in the visible and near-IR region. INTRODUCTION The optical behavior of metal nanoparticles with different geometries is widely examined in last two decades for the plasmonic resonances in the visible and infrared region. The response of a metal nanoparticle to the incident light depends on several different parameters such as metal type [1], dimensions [2, 3], geometry [4], neighboring particle distance [5] and optical constants of the host medium [6]. Variety of parameters affecting the resonance behavior brings flexibility in tuning the peak positions and makes the metal nanoparticles applicable to diverse systems such as optoelectronic devices [7, 10] and plasmon waveguides [11]. Electron Beam Lithography, being a controllable manufacturing method, helps understanding the mechanism behind the collective response of nanoparticles with well defined shapes. In this study, Au nanoparticles with rectangular shape are manufactured over Indium Tin Oxide (ITO)-coated glass substrates to investigate the optical behavior as a function of side lengths. Lengths of the nano-rods are varied between 70 and 300 nm while thickness and width are kept constant as 25 and 70 nm, respectively. Nanoparticle arrays under illumination with light having polarization parallel to one of the principal axis of the rods responded differently when electron oscillations through both axes are excited. Reflectance spectra obtained in the visible and near IR region of the electromagnetic spectrum yielded promising results for use of rod-like particles as optically switched intensity modulators.
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