Efficient 1D and 2D selfocusing and quasi-steady-state solitons in BSO crystals without background illumination

Summary form only given. Among photorefractive materials, Bi/sub 12/SiO/sub 20/ (BSO) has the peculiarity of showing strong optical activity (about 45/spl deg//mm). Previous investigations demonstrated that such a regime denies soliton formation in this material because of a not complete nonlinear compensation of the diffraction in both the polarisation components, nevertheless we have observed and characterised, at really low signal intensity and at zero background, really efficient 1D and 2D self focusing. These transient conditions survive for a few seconds. In this case, contrarily to the steady-state case, no background is present particular selfocusing regimes behave as quasi-steady state solitons for which the transversal shapes remain almost constant along 1-2 diffraction lengths. Peculiarity of this selfocusing is the unavoidably soliton destruction induced by any background presence.