Abstract.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
The interaction of a laser with potassium atoms is investigated by monitoring the
radiation of the 5P–4S transition under the two-photon 4S1/2 –6S1/2
excitation. For low number density and laser intensity the observed radiation is
parametric axial or conical, depending on the laser detuning from the
two-photon resonance, the number density and the laser intensity. With
increasing laser intensities and number densities, an additional delayed
component is observed corresponding to a two-step four-wave mixing
with the participation of internally generated photons. A two-photon
quantum interference effect is observed for the total (axial and conical)
parametric four-wave mixing emission. A deviation from the proposed
model under certain experimental parameters is attributed to a two-step
four-wave mixing and the phase mismatch of the parametric processes.
Potassium 4S1/2–6S1/2 two-photon excitation initiates the emission of several
internally generated photons. For the first time two emission lines, one close to
and one below the potassium 4P3/2 level, are reported for low pumping intensity.
Radiation emitted below the 4P3/2 level is due to a parametric four-wave mixing
process that uses the photons emitted at the 5P3/2–4S1/2 transition and a
two-step four-wave mixing process generates the line emitted close to the 4P3/2
level.
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