Effects of ionization and cascade decay on two-photon two-level interactions

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Abstract
We extend the semiclassical two-photon two-level model to include upper-level ionization and upper-level decay via an intermediate level. We examine the effects of these relaxation mechanisms on multiwave interactions, finding the generalized one- and two-wave two-photon absorption coefficients and the three-wave coupling coefficient. These mechanisms cause asymmetries to appear in the absorption spectra in addition to those caused by Stark shifts. The Stark-shift term becomes more complicated and fails to vanish even when it does in the simpler model. For recombination rates and ionization decay rates comparable to the coherence decay rate, coherent dips fill in.