Nonlinear Optical Polymers Derived From Organic/Inorganic Composites
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INTRODUCTION The development of practical materials for second-order nonlinear optical (NLO) applications, such as frequency doubling and electro-optic (EO) modulation, requires simultaneous optimization of their many critical properties. Large second-order optical nonlinearity, its long-term stability at elevated temperatures, and low optical losses are the most desirable features of materials for such types of applications [1,2]. A number of polymeric materials have been reported to surpass both the second harmonic and linear EO coefficients of the traditional inorganic NLO materials such as lithium niobate and potassium dihydrogen phosphate [2]. Second-order NLO properties are present in the polymer when the NLO chromophores are aligned in a noncentrosymmetric manner by the poling technique [3]. In order to prevent the randomization of the poled (aligned) NLO molecules in a polymer matrix, NLO chromophores are usually incorporated into a polymer which has a high glass transition temperature (Tg). This is due to the fact that the molecular motions of NLO chromophores are closely associated with the Tg of the polymer [4]. Further, enhanced temporal stability of second-order NLO properties in a poled polymer can be obtained when a certain degree of crosslinking is introduced [5-10]. Long term stability of second-order nonlinearities in a number of different crosslinked polymeric systems at temperatures higher than 100 'C has been reported earlier [6,9,10]. Sol-gel technology provides an attractive route to the preparation of a three-dimensional inorganic network [11]. In addition, sol-gel materials have been processed to lead to transparent and low loss glasses [12,13]. The basic sol-gel process involves the sequential hydrolysis and polycondensation of silicon alkoxide (Scheme 1) at temperatures that are commonly used to 531 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
process polymeric materials [11]. A three-dimensional network of inorganic/organic composites can be prepared by this process without risking the degradation of organic components [14]. By arranging different combination of the starting monomers or polymers, the properties of the multicomponent polymer system (i. e. composites) can be tailored. In light of this, it is feasible that the organic/inorganic composites with the hybrid characteristics of high NLO susceptibility, low optical loss, dense threedimensional network and high glass transition temperatures, can be achieved. hydrolysis =SiOR + HOH
=SiOH _ + ROH
water condensation -SiOH+ HOSi-
-SiOSi-= + HOH
alcohol condensation -SiOH+ ROSi-
-SiOSi-= + ROH
Scheme 1. The reaction mechanism of the sol-gel process.
Several types of NLO organic/inorganic composites, which have been developed in our laboratory, provide new angles and methods to the design of second-order NLO materials. They are described as follow.
GUEST ORGANIC CHROMOPHORE / HOST INORGANIC NETWORK Organic polymeric guest/host systems (NLO dyes as guests and polymers as hosts), which have been studied
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