Investigation of internal electric field in LiNbO3 crystal with two anti-parallel ferroelectric domains by interferometric technique

Large built-in internal field is present in congruent LiNbO3 and is due to bulk dipolar defect complexes of nonstoichiometric crystals. This field influence electric field switching of domains as well as other optical and electrical properties of crystals. Moreover it's time-temperature dependent and this feature can bias the stability of LiNbO3 based devices. We investigate how the internal field could affect the electro-optic effect. To this aim an improved interferometric technique is used to obtain a spatially resolved measurement of the electro-optic coefficient of z-cut LiNbO3 crystals. The samples are mounted in one arm of a reflective grating interferometer and resulting fringes patterns are visualized and stored by a CCD camera. Then this as-recorded data are processed by digital holographic technique in order to obtain 2D phase maps as function of the applied voltage across the crystal thickness. Hence spatial distribution of r13 is achieved for crystal either in virgin state or in domain reversed one. Moreover, samples just after poling with two anti-parallel ferroelectric domains structure have been studied. Results show that the virgin area and the reversed one have quite different r13 coefficient values. We suppose that this difference is due to the internal field, however further measurements are currently under investigation to confirm this hypothesis.