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

Abstract

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.

Photoswitching of the Second Harmonic Generation from Poled Phenyl-Substituted Dithienylethene Thin Films and EFISH Measurements

“…However the SHG signal of diarylethene chromophores in PMMA matrix progressively faded due to a loss of orientation of the dyes. This feature, which has been already reported in many other works, ,, is not surprising if the β transition of PMMA, which has been attributed to rotation of the ester side group, is considered. To avoid this problem, polystyrene was used as matrix, showing a better behavior.…”

Structure−Activity Relationship of New Nonlinear Optical Organic Materials Based on Push−Pull Azo Dyes. 3. Guest−Host Systems

“…The χ (2) time decay at 30 °C presented an initial fast relaxation process, followed by a much slower one, as generally reported in the literature. ,,,− ,, The experimental values of the second harmonic generation intensity ( I SHG ) showed very good agreement (Figure ) with a double exponential model, where the slow decay was fitted by a Kohlrausch−Williams−Watts (KWW) stretched exponential function:

4 Experimental points and KWW fitting for the 120 °C SHG time decay for the system MOD 3/PMPP.

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“…Many efforts have been devoted to the study of orientation and randomization phenomena in glassy guest−host systems, − generally using second harmonic generation (SHG) as a tool for monitoring the growth and the decay of the organization of dipoles. However, to the best of our knowledge, no useful correlation among the chemical structure of NLO-active chromophores and their orientability and stability of second-order NLO properties has been yet presented.…”

Electric Poling of 4-(Dimethylamino)-4‘-nitrostilbene in Poly(methyl methacrylate) Studied by Polarized Absorption Spectroscopy

“…This supports the two-mode relaxation mechanism. It was found ,, that the KWW function is inadequate to describe the relaxational behavior of the second-order NLO susceptibility of corona poled films and the biexponential function better fits the experimental data. Our results seem to support their conclusions.…”