Synthesis and Second-Order Nonlinear Optical Properties of New Coumaromethacrylate-Methylmethacrylate Copolymers
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SYNTHESIS AND SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF NEW COUMAROMETHACRYLATE-METHYLMETHACRYLATE COPOLYMERS R.A. HENRY,* J.M. HOOVER,* A. KNOESEN,** S.T. KOWEL,** G.A. LINDSAY,- AND M.A. MORTAZAVI** *Chemistry Division, Research Department, Naval Weapons Center, China Lake, CA 93555-6001 "**Organized Research Program on Polymeric Ultrathin Film Systems, Department of Electrical Engineering and Computer Science, University of California, Davis, CA 95616 ABSTRACT The synthesis and second-order nonlinear optical (NLO) properties of a new family of optically nonlinear coumarin dye-containing polymers is reported. Polymerizable coumaromethacrylate (CMA) monomers having various alkyl spacers (C2 to C5) between the polymerizable group and the chromophore were prepared and copolymerized with methylmethacrylate (MMA) and other vinyl monomers. The CMA copolymers had high chromophoric content (up to 18 x 1020) and glass transitions as high as 1320C. Spin cast films were prepared on BK7 substrates and oriented by corona-onset poling at elevated temperatures (COPET). The poled CMA copolymer films were characterized for spectroscopic absorption and second harmonic properties. A comparison of the nonlinear optical properties of poled CMA-MMA copolymers to poled guest-host systems is made. Relative to guest/host systems, CMA-MMA copolymer films exhibit larger and much more stable second-order nonlinear optical properties. INTRODUCTION Second-order NLO polymers have recently been the subject of extensive research. This interest is largely due to their broad spectrum of unique and useful electrical, optical, thermal, and physical properties. In general, they may have very large molecular hyperpolarizabilities (B - 100 x 10-30 esu or more) [1], high optical clarity, ultra-fast response times (- 10-14 s or less) [2], and robust mechanical integrity. In addition, many may be conveniently and inexpensively processed into thin film geometries, oriented by electric field poling, and integrated with microelectronics. Alternatively, some second-order NLO polymers may be processed using LangmuirBlodgett (L/B) monolayer deposition techniques to yield more highly ordered, ultrathin, multilayer NLO films [3-5]. Consequently, these materials are attractive for a variety of applications including optical communications and computing components, frequency doublers for lasers, and laser resistant surfaces. Two basic approaches exist for the synthesis of optically nonlinear polymers. One approach is to prepare guest-host materials by simply dissolving polarizable moieties (chromophores or dyes as the guest) in a polymeric host. The other approach is to synthesize polymers that have chromophores chemically attached as either pendent or main-chain substituents. These polymers may usually be molecularly oriented by one of the two techniques mentioned above. Additionally, these materials may be completely thermoplastic (soluble and reprocessible) after poling or L/B deposition, or crosslinked to further inhibit chromophore mobility (6].
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