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The relaxation of the second-order nonlinearity of poled chromophore-doped sol-gel thin films has been studied at different temperatures between 63 degree(s)C and 82 degree(s)C (poling temperature: 90 degree(s)C). The process has been monitored through the variations of second harmonic generation (SHG) induced in the poled film by a pulsed nanosecond Nd:YAG laser (1064 nm). The SHG decay curves have then been fitted to bi-exponentials and Kohlrausch, Williams and Watts (KWW) stretched exponentials. The better quality of the KWW fits involves that this model is more appropriate than the bi- exponential model to describe chromophore relaxation in sol- gel thin films, as has been observed for classical all- organic polymers. A numerical analysis of the KWW parameters enables a better understanding of the function meaning. The time constants obtained with both models have been logarithmically plotted in function of temperature. An Arrhenius-type linear fit has been adjusted to this representation, and a value of the relaxation time extrapolated at room temperature. The bi-exponential model yields time constants of 26 and 110,000 years, while the KWW model extrapolates to an 87 years' second-order nonlinearity lifetime with a corresponding activation energy of 49 kcal.mol-1. Decay data gathered at room temperature over a period of 17 months can be fitted with both models and yield time constants of 14 days and 13 years for the bi- exponential, and of 115 days for the stretched exponential. The extrapolated KWW fit can be corrected with a factor of 0.58 to adjust the experimental data. This reveals the possible existence of a very rapid relaxation (less than a minute for our measurement temperatures) that was not measured. Nonetheless, our work shows that second-order nonlinearity in poled chromophore-doped sol-gel thin films is stable for more than a year. This is an encouraging result for future applications in telecom industry, especially as this feature has not yet been studied and optimized in terms of material synthesis.
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