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Regular optical patterns formation and pyroelectric controlled beam self-localization in a photorefractive LiNbO3:Fe crystal

We report for the first time a regular optical patterns formation in a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. The process was controlled by pyroelectric effect resulting in self-localization of the regular optical pattern in a single light spot with reduced size and displacement relative to the input beam. The experiments were performed with the use of single mode laser radiation at 632.8 nm wavelength and with beam powers of 0.5 ÷ 10 mW. The LN:Fe sample with 0.03 wt% concentration of Fe and 10 mm lengths along beam propagation was used in the experiments. The switching of the pyroelectric effect in the process was achieved by gradual heating of the crystal in the temperature range of 10 ÷ 42 °C during 130 s. The observed phenomenon of optical patterns formation is explained with the light induced complex refractive index variation with a central minimum (negative lens) and symmetric side maxima (positive lenses) due to photovoltaic effect. Light-induced complex lens produces beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to the regular light patterns formation. The switching of pyroelectric effect results in the space-charge field modification in the crystal, asymmetric positive lens formation and light self-localization.