Strong stimulated cascade emission and coherent four-wave parametric radiations are observed. By proper observation of the excitation spectra, the cascade emission and the coherent four-wave mixing parametric processes can be identified. The effect of the buffer gas pressure on the intensity and the pulse compression of the emitted radiation were investigated. Cascade inversionless coherent radiation emission with absorption reduction and third order nonlinear susceptibility enhancement due to the presence of a strong internally generated field are discussed for the 4p–4s strong emission observed.

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It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.

Abstract.We investigate the emissions of atomic potassium excited by two photons near the 4S1/2-6S1/2 transition. Radiation near the 6S1/2-4P3/2,1/2 and 4P3/2,1/2-4S1/2 transitions are studied for the unidirectional and the bidirectional cases respectively. It is shown that the total output intensity increases considerably for the bidirectional propagation. This is attributed to the effective increase of the Doppler free excitation and the cancellation of the quantum interference effect that is associated with the multiple paths of excitation of certain states.

The Faraday effect in potassium vapour pumped by nanosecond pulses of near resonant laser radiation is studied experimentally. The input radiation with linewidth 3 cm had a central frequency v = 13055 cm' which was 12 cm' higher than 4S1, -4P3, atomic transition frequency v0 = 13043 cm' The strong spectral broadening of pump radiation allowed to observe the Faraday rotation at frequencies V±l = V0 2.5 cm' located closer to the atomic resonance where the rotation was high. The decrease of Faraday rotation from 1 500 to 20° with the increase of the intensity of pump radiation from 2lO to 1O W/ciu2 has been measured. The saturation of 4S1, -4P3, transition is considered as a cause of suppression of Faraday rotation. The contributions of one photon resonance absorption, as well as of multiphoton processes, caused by more intense off resonant part of broadened pump radiation (having frequency detuning 12 cm'), to saturation of resonant transition is discussed . The experimental results are compared with theoretical calculations performed in [10].

The collisional broadening cross sections of potassium with the rare gases He, Ne and Ar were measured, using a new absorption-polarization method, which involves the Faraday rotation near an atomic resonance. These cross sections above and below the resonance transitions 4S1/2-4P1/2,3/2 show an asymmetry of the broadenings. The interaction time of the potassium-rare gas pair was found to increase from 0.3 ps for He to 1.3 ps for Ar.