Dark hole formation in defocusing liquid media

Experimental and theoretical investigations reveal that the dark hole formation in defocusing media is a generic feature of a Gaussian beam transmission. The dark hole is the result of a strong spatial self-phase modulation which is due primarily to a thermally-induced refraction change by light absorption. The size of the dark hole was determined to show a weak linear dependence on pump power but shows a slight increase on increasing incident power. The divergence of the core region was found to be small, of the order of twice that of incident beam. While the dark hole first occurs with a single defocusing ring at a low pump power and persists for all higher pump power, the number and size of its defocusing rings increase in increasing intensity.