The use of applied electric fields the photorefractive tungsten bronze ferroelectrics

The traditional method of determining the photorefractive effective charge density is to plot the photorefractive space charge field versus the crossing angle in a two-beam coupling experiment. The difficulty with this traditional measurement technique is that the apparatus must be moved several times in order to obtain data over the sufficient number of crossing angles needed for an accurate fit with theory. Moreover, with small crossing angles the overlap between the two crossing beams can easily extend over the entire crystal, while with larger crossing angles the overlap between the two beams becomes less certain. In this paper we demonstrate an alternative method of determining the photorefractive charge density. In this approach we measure the phase shift between the optical intensity pattern in the crystal and the resulting index pattern, as a function of the magnitude of an applied d.c. field. By comparing the measured value of the d.c. field which produces a minimum phase shift with that predicted by theory the photorefractive effective charge density is found. In this case, only the magnitude of the applied field is varied and the apparatus remains fixed. The result is obtained quickly and with little error.