Z-scan measurements of photorefractive nonlinearities for a SBN:Ce crystal

In this paper, we derived the Z-scan formula for a photorefractive crystal sample under an external applied electric field. The far-field diffraction pattern of a Gaussian beam wavefront through a photorefractive crystal is calculated by considering spatial-phase perturbation induced by the space-charge field. The photorefractive drift nonlinearities and correspondingly, the electro-optic coefficients r(33), r(13), (r(23)) and the characteristic ratio n(e)(3)r(33)/n(0)(3)r(13) for a cerium-doped Sr0.61Ba0.39Nb2O6 crystal are determined from the Z-scan experiments; Z-scan with enhanced sensitivity is also realized by measuring the normalized transmittance at the off-axis position in the far field. A deviation between the off-axis Z-scan experimental results and the theoretical prediction is discussed too.