Abstract
A theoretical study of the nonlinearity reduction in a four-level system of a potassium atom in the presence of a strong nanosecond laser field, which excites the transition |4(S1 2)> <-> |6S(1/2)> with two photons, is presented. It is shown that the destructive quantum interference between the laser field and the internally generated radiations results in a linear response of the atomic path-1 (|4S(1/2)> <->| 6S1/2 <-> |5P(3/2)> |4S(1/2)>) emitted parametric fields. For sufficiently high laser intensities and/or atomic densities, the path-1 emitted fields are driven into saturation, a regime accompanied by a substantial population redistribution among the states. Reasonable agreement between earlier experimental results and the theoretical ones is obtained. It is also shown that upon saturation of path-1, for low atomic density, the path-2 (|4S(1/2)> <-> |6S(1/2)> <-> | 4P(3/2)> <-> |4S(1/)2 >) is activated. A subtle interplay between laser intensity and atomic density may determine the activation of path-2. Also, it is shown that the path-2 emission |6S(1/2)> <-> |4P3/2 > is an amplified spontaneous emission process which induces a |4P(3/2) > <-> |4S(1/2)> emission, without population inversion, in a cascade scheme.