Saturation and population transfer of a two-photon excited four-level potassium atom

A theoretical study of the nonlinearity reduction in a four-level system of potassium, atom in the presence of a strong nanosecond laser field, which excites the transition 1/2 1/2 4S 6S ↔ 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 (1/2 1/ 2 3/2 1/2 5P 4S 6S 4S ↔ ↔ ↔) 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 substantial population redistribution among the states. A 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 (1/2 1/2 3/2 1/2 4P 4S 6S 4S ↔ ↔ ↔), 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 1/2 3/2 4P 6S ↔ is an amplified spontaneous emission process which induces a 3/2 1/2 4P 4S ↔ emission, without population inversion, in a cascade scheme.