Axial coherent emissions controlled by an internal coupling field in an open four-level potassium system

Abstract

We present a theoretical model interpreting the experimental results observed under strong two-photon ns laser excitation of the $\left| {6{{\text{S}}_{1/2}}} \right\rangle$ potassium atomic state, where emissions near the $\left| {6{{\text{S}}_{1/2}}} \right\rangle \leftrightarrow \left| {4{{\text{P}}_{3/2}}} \right\rangle$ and $\left| {4{{\text{P}}_{3/2}}} \right\rangle \leftrightarrow \left| {4{{\text{S}}_{1/2}}} \right\rangle$ transitions were experimentally observed. It is shown that the $\left| {6{{\text{S}}_{1/2}}} \right\rangle \leftrightarrow \left| {4{{\text{P}}_{3/2}}} \right\rangle$ emission initially grows nonlinearly with pump intensity, while subsequently saturates and enhances the generation of radiation near the $\left| {4{{\text{P}}_{3/2}}} \right\rangle \leftrightarrow \left| {4{{\text{S}}_{1/2}}} \right\rangle$ transition. It is found that a coherent manipulation of an open four-level system is possible by an internally generated, saturated coupling field, despite the energy decay to the continuum. The efficiency of the proposed coherent control method is managed by adjusting the pump intensity and potassium density. Finally, a general control scheme is discussed in which an external pump and an internal coupling field determine the system’s response in a cascade scheme.