We report on steady-state dark spot formation in the center of the far field of a divergent Gaussian beam transmitted through a thin absorbing defocusing liquid medium. The size of the dark region is found to be approximately conserved with increase of incident pump power. Theoretical analysis shows a good quantitative agreement with our experimental measurements, establishing the physical origin of such a pattern formation to arise from interplay between wavefront curvature of the input Gaussian beam and strong spatial self-phase modulation by thermally-induced refractive index change in the medium, which we show to be dominated by longitudinal thermal diffusion.
|Number of pages||4|
|Publication status||Published - 15 Jun 1997|