The expression for the light-scattering intensity is deduced using the general perturbation expansion for a stationary solution of the Liouville equation. The principal importance of translational symmetry for coherent spectroscopy is rigorously shown. The expressions for the degenerate four-wave mixing (DFWM) signal intensity as well as for its polarization are derived in the case of fast translational relaxation (Dicke narrowing). The instructive relationship between the index of refraction, the absorption spectrum, and the DFWM is established.
We analyze a four-wave-mixing (FWM) scheme in a five-level atomic system based on electromagnetically induced transparency (EIT). We show that EIT suppresses both two-photon and three-photon absorptions in the FWM scheme and enables the four-wave mixing to proceed through real, resonant intermediate states without absorption loss. The scheme results in a several orders of magnitude increase in the FWM efficiency in comparison with a recent scheme [Phys. Rev. Lett. 88, 143902 (2002)] and may be used for generating short-wavelength radiation at low pump intensities.
We consider effects which occur when four-wave sum frequency generation and multiphoton ionization are induced by lasers tuned near a three-photon resonance and simultaneously near or at a dipole allowed four-photon resonance. In studies with unfocused laser beams, if the phase mismatch of the generated four-wave-mixing field is large and the related two-photon resonance for the absorption of a four-wave-mixing photon and a laser photon results in strong absorption of the four-wave-mixing field, a coherent cancellation occurs between the pumping of the resonance by two- and four-photon processes. This interference effect occurs when the first laser is tuned on either side of the three-photon resonance and \ensuremath{\Vert}\ensuremath{\Delta}
krL\ensuremath{\Vert}\ensuremath{\gg}1, where \ensuremath{\Delta}
kr is the mismatch and L is the length of the path of the laser beams in the gas. With focused laser beams large differences occur between ionization with unidirectional beams and with counterpropagating laser beams when \ensuremath{\Vert}\ensuremath{\Delta}
krb\ensuremath{\Vert}\ensuremath{\gg}1, where b is the confocal parameter of the focused laser beams. Strong absorption of the four-wave-mixing field is shown not to be necessary for strong destructive interference with focused laser beams when the phase mismatch is large. This work also suggests an explanation for earlier experiments where the presence of a four-photon resonance enabled the generation of third-harmonic light in a positively dispersive wavelength region. We argue that this process can occur when the laser used to achieve the four-photon resonance is focused on the small z (z is the coordinate in the direction of propagation) side of the focal point of the laser responsible for the third-harmonic generation. In this way the absorption of the third-harmonic light on one side of the focal point is made much stronger than on the other side. Correspondingly, contributions to the third-harmonic field from the two sides of the focal point are unequal and cannot undergo complete destructive interference as would otherwise occur in a positively dispersive region.