A NEW SELF-DIFFRACTION PATTERN-FORMATION IN DEFOCUSING LIQUID-MEDIA

We report on steady-state dark spot formation in the center of the far field of a divergent Gaussian beam transmitted through a thin absorbi ng defocusing liquid medium. The size of the dark region is found to b e approximately conserved with increase of incident pump power. Theore tical analysis shows a good quantitative agreement with our experiment al measurements, establishing the physical origin of such a pattern fo rmation to arise from interplay between wavefront curvature of the inp ut Gaussian beam and strong spatial self-phase modulation by thermally -induced refractive index change in the medium, which we show to be do minated by longitudinal thermal diffusion.