Second Order Nonlinear Optics and Polymer Physics of Corona Poled, Doped Polymer Materials
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SECOND ORDER NONLINEAR OPTICS AND POLYMER PHYSICS OF CORONA POLED, DOPED POLYMER MATERIALS HILARY L. HAMPSCH*, JIAN YANG*", GEORGE K. WONG**, JOHN M. TORKELSON* *Department of Materials Science and Engineering; **Department of Physics and Astronomy; ***Department of Chemical Engineering; Northwestern University, Evanston, Illinois 60208. INTRODUCTION Doped glassy polymers have been examined as potential materials for nonlinear optical (NLO) device applications[ l-Il1]. In these systems, dopants with excellent nonlinear optical capabilities are dispersed in glassy polymer matrices with good physical properties to make versatile and efficient NLO materials[1-4]. The optical technique of second harmonic generation (SHG), conversion of light of frequency to to light of frequency 2o, is performed as a function of time to examine the temporal stability of NLO dopant orientation in the polymer matrix[I-4]. Through the use of poling, the NLO dopants are aligned into the noncentrosymmetric orientation required for SHG to occur[1-4,7]. Polymeric NLO materials have a number of advantages over the current commercial inorganic crystals, including ease of fabrication and processability, low laser damage, low cost, and excellent chemical and physical resistance[8-10]. Due to the relaxation behavior characterizing glassy polymers even at temperatures well below the glass transition temperature Tg, the dopants can disorient as a function of time following poling[1-4,12,13]. This results in a loss of optical performance with time. The purpose of this work is to examine the basic polymer physics that govern the temporal stability of the dopant orientation and disorientation and related optical behavior as a function of the local microenvironment surrounding the NLO dopants. Systems studied include bisphenol-A-polycarbonate (PC), polystyrene (PS) and poly(methyl methacrylate) (PMMA) doped with well characterized NLO dyes such as 4-dimethylamino-4'nitrostilbene (DANS) and 4-amino-4'-nitroazobenzene (or disperse orange 3, D03). Second order NLO effects are only observed in systems when the dopants are oriented noncentrosymmetrically in the polymer matrix[8]. Contact[l] and corona [2-7] poling, both involving large electric fields imposed across the polymer film, are used to orient the NLO chromophores. Corona poling creates the electric field via discharge-generated ions that are accelerated towards the film surface. The bottom of the sample is grounded, so a large field is produced across the film normal to the surface and the highly polar dopants with large dipole moments align in the field direction. Poling can be performed at any temperature as long as the polymer segments have enough mobility (or the polymer has enough local free volume) to allow the dopants freedom to rotate along the electric field vector[l]. When the orienting field is generated by a corona discharge, significant SHG signal can be observed even when poling doped PMMA films at room temperature. This is not observed when using contact poling. When the applied fiel
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