Optically Controlled Optical Modulator using a Self-Assembled 2D Plasmonic Crystal
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Optically Controlled Optical Modulator using a Self-Assembled 2D Plasmonic Crystal Yi Lou and John F. Muth Department of Electrical and Computer Engineering, North Carolina State University Raleigh, NC 27606, U.S.A ABSTRACT The surface plasmon enhanced transmission of light though a plasmonic crystal provides a novel approach for fabricating an optical modulator. The extraordinary transmission passing though these patterned metallic films is very sensitive to the surface dielectric environment. In this study, hexagonal lattice plasmonic crystals were fabricated with a self-assembly technique. Arrays of gold nano-holes or bumps with 500/600 nm periodicity were used to test the sensitivity of plasmon resonance wavelength for liquids and polymers with different dielectric constants. A nonlinear optical polymer P3HT coated onto the plasmonic crystal and pumped with 475 nm laser was found to modulate the transmission of a normally incident red light at 670 nm. INTRODUCTION Photonic crystals are periodic structures where the dielectric constant varies with size scales comparable to the desired wavelength of light. This has enabled numerous optical devices [1,2]. More recently, there has been interest in fabricating similar devices using metallic structure where the dielectric constant has an imaginary part and the free electrons can be excited by the oscillating electric field of the incident light. At suitable optical frequencies this results in collective oscillations of electrons called plasmons. Engineering plasmonic crystals where the interactions of plasmons and light can be designed and controlled by scaling structures comparable to the wavelengths of the plasmons potentially enables new types of optical sensing [3], imaging [4], and modulation devices [5,6]. In thin films, Surface Plasmon Resonance (SPR) occurs when the momentum matching conditions for light incident on a thin metal film are met and the incoming photon excites an oscillation of the electron gas that travels along the plane of the surface called a surface plasmon. For isolated nanostructures such as metallic nanoparticles, Localized Surface Plasmon Resonance (LSPR) occurs when the oscillations in electron density are confined to the particle. LSPR is strongly related to the shape of the metal particles, which determines coupling efficiency. Plasmonic crystals are constructed by patterning the metal surface, usually gold or silver, with periodic nano-hole or nano-bump arrays. The periodicity of the array determines the resonance frequency of SPR, while the size and shape of single nano-hole/bump controls the photon coupling efficiency. In this study, a plasmonic crystal was constructed using arrays of gold nano-holes or arrays of nano-bumps such that surface plasmon resonance effects imposed by periodicity of the array can be utilized to control the transmission rate of a normal incident light. The resonant condition of SPR is sensitive to the dielectric constant of the material just above the metal surface. Poly(3-hexylthiophene) (P3HT
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