Effects of the multiple internal reflection and sample thickness changes on determination of electro-optic coefficient values of a polymer film

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New nonlinear optical (NLO) active organic materials are appealing candidates for optoelectronic and photonic technologies. For the evaluation of new NLO polymer materials for applicability in the mentioned technologies, the most important criteria are their electro-optic (EO) coefficients. We have implemented the Mach–Zehnder interferometric (MZI) method for the determination of EO coefficients of thin organic films. Despite the fact that other multiple optical methods for the determination of thin film EO coefficients are known, the MZI method has been chosen because this particular technique has high sensitivity to phase and intensity modulations in the sample arm of an interferometer and allows one to determine independently both thin film EO coefficients, r13 and r33. In addition to the drawbacks described earlier we demonstrate that some other effects like electrostriction and multiple internal reflections in the sample have a considerable influence on light intensity at the MZI output. Taking into account these effects we have performed numerical simulations of the EO effect caused MZI output changes or modulation depth at different incidence angles using the Abeles matrix formalism. We can show that the modulated signal at the MZI output is highly dependent on the sample structure and is mainly governed by the effects mentioned above. For analysis of modulated signal components and determination of EO coefficients of a thin polymer film, a series of experiments was carried out on PMMA + DMABI 10 wt% samples.