Mechanism of refractive-index change in a Ce:SBN single crystal illuminated by nominal homogeneous light

We report a series of experiments demonstrating that the refractive index n(e) of a single crystal of Ce:SBN increases when the crystal is illuminated by a nominal homogeneous Ar laser beam. The increase is larger than that owing to the pure temperature rise, which is equal to that occurring in a Ce:SBN crystal illuminated by the same laser beam. We discovered that the difference between these Deltan(e)'s owing to the light and to the heat caused by this light is closely related to the crystal's history of illumination; the more it is illuminated, the less the difference is. Also, when the illumination exceeds the threshold volume for the first time, the refractive index will increase to be linear. We believe this to be the first report of these phenomena; we have introduced a theoretical explanation.