Dynamic transition from self-defocusing to self-focusing in photovoltaic LiNbO 3 :Fe crystals

In this paper we demonstrate for the first time to our knowledge that the LiNbO₃:Fe crystal has the dynamic behavior of transition from self-defocusing to self-focusing under certain experiment conditions. It represents that, after a period of illumination, the peak intensity transmission increases and the FWHM becomes narrower gradually. By comparing the experiment result under different conditions, we believe that his kind of transition exists universally in LiNbO₃:Fe crystal. We think that its mechanism is based on the following. At the beginning of illumination, due to the Gaussian distribution of the incident intensity, photorefractive effect forms a negative lens in illumination area, making the light beam widened and diverged. While when the change of the refractive index reaches saturation, the negative lens effect disappears. A double phase conjugate Fabry-Perot interference cavity is formed between the input plane and output plane of the crystal. The multiple-beam interference causes the output beam brighter and thinner.