ROTATIONAL BROWNIAN-MOTION OF CHROMOPHORES AND ELECTRIC-FIELD EFFECTSIN POLYMER-FILMS FOR 2ND-ORDER NONLINEAR OPTICS

Rotational dynamics of nonlinear optical chromophores embedded in amor phous polymer films were studied using second harmonic generation. Cor ona poling was used to orient the chromophores into the bulk noncentro symmetric structure required to observe second-order nonlinearity. Ele ctric field effects were examined by simultaneously measuring the seco nd harmonic signal (during and following poling) and surface voltage d ecay (following poling). It is found that for short times the residual field following poling retards chromophore reorientation. A mathemati cal model that describes the rotational Brownian motion of chromophore s in a polymer matrix is developed to simulate the field-dependent beh avior. The electric field effects can therefore be deconvoluted from t he Brownian motion to reveal information concerning local mobility in polymers. Further applications of the model in distinguishing the post -poling electric field effects and in computing the local free volume and viscosity are discussed. A first attempt is made to realize the co ntributions of the residual surface voltage, field-induced bulk charge s, and thermally injected charges to the rotational motion of the chro mophores. The magnitude of the local free volume and the local viscosi ty-temperature behavior in a doped poly(methyl methacrylate) system ar e estimated and compared with those predicted by the DoolittIe-William s-Landel-Ferry equation.