Surface Plasmon Enhanced Transmission with Self-assembled Hexagonal Nanohole Arrays
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Surface Plasmon Enhanced Transmission with Self-assembled Hexagonal Nanohole Arrays Yi Lou1, Nathan Westcott2, John Mcglade1, John F. Muth1 and Muhammad Yousaf2 1
Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27606 2 Department of Chemistry, University of North Carolina-Chapel Hill, Chapel Hill, NC 27599 ABSTRACT The optical properties of hexagonal nanohole arrays in gold films are investigated. Nanosphere lithography combined with reactive ion etching has been applied as a low cost method to fabricate nanohole arrays with hexagonal symmetry where the size and spacing of the holes can be independently controlled. In this study, the spacing between the nanoholes is 600 nm with the hole diameter varied between 450 and 250 nm. The transmission spectra of the surface patterns with different film thickness are collected with normally incident light. The color of the reflected light from the nanohole array was found to change from green to red as the diameter of the holes was reduced. One application of these films is to study cell adhesion to small areas with controlled size. We explore the possibility of making isolated cell adhesion dots by chemically modifying the nanohole area. Swiss 3T3 cells were adhered onto the patterned surface and imaged using environmental SEM and fluorescent microscopy. INTRODUCTION Surface Plasmon Resonance (SPR) occurs when the momentum matching conditions for light incident on a thin metal film are met and the electric field of the photon results in an oscillation of the electron gas that travels along the plane of the surface, called a surface plasmon. Localized Surface Plasmon Resonance (LSPR) occurs in nanoscale metal particles where the oscillations in electron density are confined to the particle. Recently, there has been interest in structuring and patterning surfaces such that the surface plasmon resonance can be engineered to occur at specific wavelengths or engineered to be suitable for novel device applications. Patterning a thin gold film with subwavelength apertures for example has led to the observation of extraordinary transmission.[1] The application of surface plasmon resonance (SPR) for bio-chemical sensing is well known and is commercially available in instruments such as those made by Biocore.[2,3] These instruments are used to detect single monolayers of proteins, or other molecules that bind to a prepared gold surface. Instruments rely on a prism in the Otto or Kretschmann configuration to meet the momentum matching conditions, which increases the complexity of the instrument and its size. LSPR allows the surface plasmon to be excited with light normally incident to the sample without any bulk optics and has also been used to sense biomolecules. In this study we are interested in preparing a patterned thin metal film on the nanoscale such that surface plasmon effects can be exploited at normal incidence under an optical microscope and thus may potentially be used by biologists for studying cell substrate
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