Abstract

If a principal atomic transition (dipole allowed from the ground state) is resonantly driven by odd-multi-photon excitation or by stimulated hyper-Raman scattering, a nonlinear polarization is generated at the transition frequency. This polarization serves as a source for generation of a multi-wave mixing field which can greatly modify the atomic response even though the generated field may not produce observable photons (the medium can be quite opaque at the resonant frequency). Under circumstances where the parameters κz/Γ ≫ 1, a number of dramatic effects on odd-photon mediated processes can ensue as a result of the internally generated wave-mixing fields, (κ = πNe²F_o,n /mc, where N is the number density, F_{0, n} is the oscillator strength between state | n> and the ground state | 0 >, z is the path length in the medium and Γ is the laser bandwidth.) In some instances the effects occur even at quite low pressures. Excellent agreement between theory and experimental studies in Xe and in metal vapors is illustrated for a number of predicted effects including complete suppression under unidirectional pumping and strong shifting under multidirectional pumping of resonance lines^[1] and hyper-Raman emissions^[2] with single and multi-laser sources. A comprehensive treatment of the problem yields the shifts and the suppression effects together as closely related aspects of the same problem.^[3,4] The experimental dependencies of the interference-related effects on gas number density, oscillator strength, wavelength combination and relative propagation directions of pump laser beams are demonstrated. Confirmed also is the fact that, contrary to most other nonlinear effects, none of the features (in the semi-classical regime) depends on the intensity of the driving laser field.

© 1992 IQEC