Optical depoling

Together with PAP, photo-induced depoling (PID) is another interesting phenomenon at the interface of photochemistry and organic nonlinear optics. Indeed, PID of poled polymers occurs when NLO chromophores, which are oriented in a polar manner, undergo photoisomerization without applied dc field. The chromophores lose their initial polar orientation after photoisomerization and reorientation in azimuthal directions around the initial polar axis, thereby erasing FID has been observed both by photo-induced destruction of EO Pockels and by second harmonic generation. The first published PID experiments have been reported for DRl in PMMA, and the theory of PID is discussed in detail in reference 25. 

Hayden, L.M., Brower, S.C., Strutz, S.J. Pressure dependence of the depoling temperature in nonlinear optical polymers. Macromolecules 30(9), 2734-2737 (1997)... 

There are, however, more subtle thermal stability requirements for the electro-optic materials described here. The electro-optic response in these polymers arises from a non-centrosymmetric orientation created during the process of poling and subsequent cooling. Thus the structures are thermodynamically unstable and are subject to reorientation with corresponding loss of response. This "depoling phenomenon" has been studied by a number of workers. The rates for this process increase dramatically as the temperature approaches Tg. For example. Fig. 5a shows on a log-log plot isothermal decay of the electro-optic response at 815 nm for MAI (32%) poled at 0.5 MV/cm for 5 minutes at T, (127 C). It is clear that the kinetics are very complex, but we may define a "half-life" for the response and plot this versus inverse temperature (°K) on a traditional Ahrennius scale as shown in Fig. 5b. This plot does appear linear. If we can extrapolate these results to lower temperatures, these data suggest a half-life of about 1... 

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