Recent Progress of Electro-optic Polymers for Device Applications
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Introduction Electro-optic (E-O) polymers have drawn great interest in recent years because of their potential applications in photonics devices such as high speed modulators and switches, optical data storage and information processing1-2. In order to have suitable materials for device fabrication, it is essential to design and develop polymeric material systems (active and passive polymers) with matched refractive indices, large E-O coefficients, good temporal and photochemical stability. 3 -8 The E-O response of an active polymer commonly arises from the electric field induced alignment of its second-order nonlinear optical (NLO) chromophore, either doped as a guest/host system or covalently bonded as a side-chain. Because of the strong interaction among the electric dipoles, the poled structure is in a meta-stable state; the poled NLO chromophores which possess large dipole moment will tend to relax back to the randomly oriented state. As a result, the stability of the poled structure strongly depends on the rigidity of the overall material system. As it might be expected, the continuous increases of the rigidity and Tg of poled polymers imposes constraints on the selection of suitable chromophores that can survive the hightemperature poling and processing conditions. To circumvent this problem, we have developed a series of chromophores that possess conformation-locked geometry and perfluoro-dicyanovinylsubstituted electron-accepting group which demonstrate both good thermal stabilty and nonlinearity. This paper provides a brief review of these highly efficient and thermally stable chromophores and polymers for device applications. Synthesis of NLO Chromophores with Enhanced Thermal Stability Nonlinearity by Using a Conformation-locked trans-Polyene Approach
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Recently, it has been shown that very large nonlinearities can be achieved by employing heteroaromatic rings which have lower aromatic stabilization energy upon charge separation, 9 -10 or by using extended polyene it-bridge systems that have strong electron acceptors. 11 Incorporation of these chromophores into polymer matrices as guest/host and their subsequent alignment by an electric field have demonstrated very large electro-optic coefficients (r33=45-55 pmV-1) at a wavelength of 1.3 gm 12 -13 . These values are significantly larger than the value of 31 pm/V for commercially available LiNbO 3 . However, the long-term alignment stability of these E-O polymer systems was limited due to the low inherent thermal stability (< 175 'C) of these chromophores that prevents them to be processed and poled at higher temperatures. We have recently developed a facile approach to synthesize NLO chromophores with both enhanced nonlinearity and thermal stability. This synthetic method combines the advantages of using a thiophene ring and a triene as efficient conjugating moities for easier charge separation, and a 2,2'-dimethyl-propyl group connected 6-membered ring system that provides a conformation-locked geometry of trans- triene to prevent the thermally ind
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