When a principal atomic transition (dipole allowed from the ground state) is driven by odd- multi-photon excitation or by stimulated hyper-Raman scattering, a nonlinear multi-wave mixing field is generated at the transition frequency. A number of dramatic effects on such odd-photon mediated processes can ensue as a result of the internally generated wave-mixing fields. Excellent agreement between theory and experimental studies in Xe and in metal vapors is illustrated for a number of predicted effects including: suppression and strong shifting of resonance lines and hyper-Raman emissions under single and multi-laser excitation. We illustrate the dependence of the interference-related effects on pressure, oscillator strength, wavelength combination and relative propagation directions of pump laser beams. Contrary to most other nonlinear effects, we show that none of the features (in the semi-classical regime) depends on the intensity of the driving laser field.
We report on experimental results of four-wave mixing processes in rubidium vapor where coherence is induced on the three-photon resonant transition from
5s to
6p states via intermediate Rydberg levels. It is shown that the use of two beams in a noncollinear configuration, i.e., $\ensuremath{\theta}\ensuremath{\ne}0$, and high atomic density unveil new features. First, the $\ensuremath{\theta}=0$ (collinear configuration) odd-photon destructive interference between the incident and generated fields is strongly inhibited for $\ensuremath{\theta}\ensuremath{\ne}0$. Second, most importantly, the observed cooperative frequency shift of the three-photon transition is strongly enhanced for small, but nonzero, values of $\ensuremath{\theta}$ due to the factor ${(1\ensuremath{-}cos\ensuremath{\theta})}^{\ensuremath{-}1}$, which is not present if the generated radiation field is not considered self-consistently in the Maxwell-Bloch equations.
In this paper, we investigate the spontaneously generated coherence (SGC) effects on the fluorescence fields intensity-intensity correlation in a resonant driven four-level Y-type atomic system. By using a strong control field, in the absense of SGC effects, strong correlation and anticorrelation fluorescence photons can be produced from two ladder transitions in this system respectively. However, in the presence of SGC effects, the fluorescence fields correlation properties are reversed for the two transitions. The above phenomena can be traced to the quantum destructive or constructive interference in terms of dressed states.