Pulse poling of side-chain and crosslinkable copolymers

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Abstract
The growth of the macroscopic second-order nonlinear optical properties of side-chain and crosslinkable copolymer species is theoretically modeled. The model describes the rotational diffusion of nonlinear diazo-dye dipoles, in the presence of crosslinking sites randomly distributed in the copolymer matrix, under application of a periodic electric poling field. Solutions for the fundamental equations, describing the time-dependent orientational probability distribution for the crosslinked dipoles, have been obtained in the frame of two complementary approximations. Such distributions have been used in order to evaluate the temporal growth of the second-order nonlinear optical properties and their asymptotic value when saturation of the crosslinking process is attained. The two approximations are shown to give equivalent results, in the range of parameters where both are valid. The influence of the poling electric field frequency and of the volume density of crosslinking sites on the asymptotic second-order nonlinear optical properties is discussed. The results of the theoretical model are compared to the experimental ones, obtained for two copolymers species: A Disperse red 1 side-chain copolymer and a crosslinkable evolution of the latter, Red acid magly. In the experiments the r33 component of the electro-optic coefficient is measured by means of a modified electro-optic ellipsometry setup. The comparison shows a good agreement between experimental and numerical results. The model can be used in order to find the best values of the copolymer parameters, necessary in order to maximize the final second-order nonlinear optical properties.