Analysis of dark spot formation in absorbing liquid media

A theoretical analysis based on coupled field-matter equations is given to describe the recently observed phenomenon of a central dark spot formation of a Gaussian beam transmitted through an absorbing defocusing liquid mediu m. We find that such a pattern formation, which is accompanied by normal de focusing rings in the far field, originates from interplay between the wave -front curvature of the Gaussian beam and strong spatial self-phase modulat ion arising from thermally induced refractive index change in the medium. R esults of numerical analysis for a thin medium are shown to be in a good qu antitative agreement with our experimental findings. Further, the dark spot formation is also predicted by using a focused Gaussian beam and self-focu sing medium.