Abstract

Self-focusing and spatial ring formation have been predicted in strongly absorbing media of two-level atoms. The requirement for observation of this effect is that the absorption-length product must be greater than the ratio of the input intensity to the saturation intensity and both greater than unity. We have observed a spatial ring formation of an argon-ion laser beam traversing a sample of pink ruby pumped above resonance. In addition, we observed the development of on-axis nulls in the intensity profile. Experiments were also performed on the low-frequency side of the line with a rhodamine 6G dye laser. Ruby is a three-level system that can be modeled as a two-level system with a low saturation intensity because of the long lifetime of the Kramers doublet. The ruby sample was 7 cm long with an absorption coefficient of 0.75 cm⁻¹. With an input intensity of 1 kW/cm² we are ~2.5 times the saturation intensity of ruby, thereby satisfying both conditions where a spatial ring formation would be expected. We also observed intensity-dependent beam fission leading to an off-axis beam that tracked the c axis and was of the opposite polarization of the main beam. Experiments performed on a 1-cm sample of ruby exhibited no rings, and a second beam was not observed confirming the predicted conditions and excluding any interface effects.

© 1988 Optical Society of America