FABRY-PEROT INTERFEROMETRY APPLIED TO THE STUDY OF PIEZOELECTRIC AND ELECTROOPTICAL PROPERTIES OF A POLED NLO POLYURETHANE

An improved Fabry-Perot interferometric technique for the in situ meas urement of electrooptical coefficients, piezoelectric coefficient and birefringence of nonlinear optical (NLO) polymer films has been develo ped. The measurements can be done conveniently on polymer films mainta ined under inert gas or vacuum and at temperatures up to 200-degrees-C . The technique has been applied to the characterisation and disorient ation study of an inhouse synthesised azo dye-containing polyurethane exhibiting a glass transition temperature at 143-degrees-C and having an electrooptical coefficient r33 of 18 pm/V at 830 nm after poling un der 100 V/mum. The uncertainty on the electrooptical coefficients was found to be 4% on r13 and 8% on r33 but raises up to 25% for the piezo electric coefficient d which is negative and about 10% of r33. For pol ing fields E(p) less than 50 V/mum, r13, r33 and d vary linearly with E(p) but r33 increases more than linearly for higher poling fields. Th e ratio r33/r13 is greater than 3 and raises up to 5 after poling unde r 120 V/mum. The rate of disorientation of the chromophores in the pol ymer matrix at elevated temperatures follows approximately stretched e xponentials and is weakly dependent on the poling field in the range 2 5-92 V/mum. Our experimental results are compared to values deduced fr om various theoretical models such as the isotropic and the Van der Vo rst-Picken models. None of them are entirely satisfactory.