Fabry-Pérot interferometry applied to the study of piezoelectric and electro-optical properties of a poled NLO polyurethane

Abstract

An improved Fabry-Pérot interferometric technique for the in situ measurement of electrooptical coefficients, piezoelectric coefficient and birefringence of nonlinear optical (NLO) polymer films has been developed. The measurements can be done conveniently on polymer films maintained under inert gas or vacuum and at temperatures up to 200°C. The technique has been applied to the characterisation and disorientation study of an inhouse synthesised azo dye-containing polyurethane exhibiting a glass transition temperature at 143°C and having an electrooptical coefficient r 33 of 18 pm/V at 830 nm after poling under 100 V/μm. The uncertainty on the electrooptical coefficients was found to be 4% on r 13 and 8% on r 33 but raises up to 25% for the piezoelectric coefficient d which is negative and about 10% of r 33. For poling fields E p less than 50 V/μm, r 13, r 33 and d vary linearly with E p but r 33 increases more than linearly for higher poling fields. The ratio r 33/ r 13 is greater than 3 and raises up to 5 after poling under 120 V/μm. The rate of disorientation of the chromophores in the polymer matrix at elevated temperatures follows approximately stretched exponentials and is weakly dependent on the poling field in the range 25–92 V/μm. Our experimental results are compared to values deduced from various theoretical models such as the isotropic and the Van der Vorst-Picken models. None of them are entirely satisfactory.