Abstract

We report on an experimental and theoretical investigation of a striking new phenomenon of dark hole formation in the center of a Gaussian laser beam transmitted through an absorbing defocusing medium. The transverse intensity distribution of the transmitted light in this region, observed in the far field, is shown to be the “negative’’ image of the input beam profile; with increasing pump power, the transmitted beam profile transferring from a Gaussian distribution to one with a central dip, the depth of which initially increases with pump intensity and then becomes invariant, the transmitted signal strength in the central dip (dark spot) falling to the order of 90% of the peak transmitted light. The size of the dark spot is found to exhibit a weak linear dependence on pump power, showing a slight increase on increasing incident power. The divergence of the core region is small, of the order of twice that of incident beam. Further, we find that at low pump power the dark hole first occurs accompanied by a single defocusing ring and persists for all higher pump power, whereas the number of defocusing rings and their size dramatically increase with increasing intensity. Figure 1(a) is a representative example of the central dark hole surrounded by 12 defocusing rings from collimated cw emission from Argon ion laser (5145 Å) of power P = 1.6 W transmitted through a 3 mm thick cell of a liquid dye material (Rhodamine 6G in methanol) with an absorption coefficient a = 4.49 cm^-1.

© 1996 IEEE