OPTIMIZED POLING OF NONLINEAR-OPTICAL POLYMERS BASED ON DIPOLE-ORIENTATION AND DIPOLE-RELAXATION STUDIES

Nonlinear optical polymers contain molecular dipoles with very large h yperpolarizabilities in a glassy polymer matrix. Two typical examples- a guest-host system with dispersed polar dye molecules and a side-chai n material with chemically attached molecular dipoles-were investigate d by means of poling experiments, dielectric spectroscopy, thermally s timulated depolarization, and electro-optical thermal analysis. The di electric behavior of both polymers can be described by the phenomenolo gical Havriliak-Negami equation, and the existence of master curves fo r both materials demonstrates the validity of the time-temperature sup erposition principle above the respective glass transitions. Temperatu re-dependent mean relaxation times and relaxation-time distributions c alculated from the dielectric data allow for an optimization of poling times. The dielectric relaxation strengths obtained from poling curre nt and field, from dielectric measurements, and from thermally stimula ted depolarization are in very good agreement and thus represent a use ful measure of the polarization in poled polymers. From the temperatur e dependence of the polarization, optimal poling temperatures may be d erived. Electro-optical thermal analysis yields the same temperature-s tability curves as thermally stimulated depolarization and is therefor e a valuable tool for investigating the stability of poled polymers, e specially since it is not sensitive to charge effects. Optimal poling fields and currents must be selected as a compromise between high dipo le mobilities (short relaxation times) and low bulk conductivities.