Observation of continuous-wave on-resonance “self-focusing”

Abstract

We report what to our knowledge are the first observations of an increase in the on-axis intensity of an intense cw on-resonance beam resulting from nonlinear absorption and diffraction during its propagation through a highly absorbing medium (sodium vapor). This "self-focusing" is not a self-lensing; instead it is modeled well by placing an aperture part way through the cell: The stripping by the aperture approximates the effect of the nonlinear absorption, and then Fresnel diffraction results in on-axis minima and maxima.