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.