The propagation characteristics of a focused laser beam in a SBN:75 photorefractive crystal strongly depend on the signal-to-background intensity ratio (R=Is/Ib) under reverse external electric field. In the range 20>R>0.05, the laser beam shows enhanced self-defocusing behavior with increasing external electric field, while it shows self-focusing in the range 0.03>R>0.01. Spatial solitons are observed under a suitable reverse external electric field for R=0.025. A theoretical model is proposed to explain the experimental observations, which suggest a new type of soliton formation due to "enhancement" not "screening" of the external electrical field.
We have observed the formation of photorefractive spatial quasi-stead-state solitons in SBN:75 crystal by electric field with white beam as background and signal beam intensity of μW magnitude. It was found that the electric field had obvious effect on the characteristics of laser beam propagation in crystal. It showed a performance of self-focusing that the output beam from the crystal is proportional to the voltage of the external electric field along the c-axis of the crystal.At the voltage of 900V(1800V/cm), (2+1) -dimensional bright spatial solitons formed. While applied voltage along the reverse of c-axis, the diffraction of spot on the output face of the crystal appeared obviously as a performance of self-defocusing.
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