Polar order and relaxation of second order nonlinear optical susceptibility in an electric field polarized amorphous polymer

The dependence of the second harmonic generation (SHG) signal of an amorphous polymer system containing nonlinear optical (NLO) chromophores on the polarizing electric field is investigated. It is shown that under a weak poling field condition, the SHG intensity is proportional to Ep2, provided that the polarizing electric field Ep is greater than a threshold field Es. Above Es, the macroscopic polar orientational order is established. The lengthening of the relaxation time for polar orientational order with increasing Ep is also observed. Theoretical analysis shows that the relaxation time is proportional to the polar order initially induced by the polarizing field. Experiments have been carried out to confirm the theoretical prediction.