Pyroelectrical investigation of the dipole orientation in nonlinear optical polymers during and after photoinduced poling

Several of the chromophore molecules considered for use in nonlinear optical polymers exhibit photoisomerization. This photoinduced transformation from one isomer to another changes the shape of the dipolar molecule and thus often enhances its mobility within a polymer matrix. Consequently, the molecular dipoles can sometimes be oriented by means of an electric field even well below the glass transition temperature of a nonlinear optical polymer if photoisomerization can be exploited. This effect forms the basis of photoinduced poling which was investigated in detail and also used for the preparation of poling patterns with inverted dipole orientations along the thickness direction of a polymer film. By means of pyroelectrical measurements, the photoinduced poling process was examined in situ and the resulting nonuniform dipole orientation profiles were probed. From the combination of pyroelectrical thermal analysis and thermally stimulated depolarization, a complete description of the relaxation behavior in polymer samples after photoinduced poling was obtained.