Propagation effects in two-color coherent-control processes

We have investigated a two-color coherent-control process at high densities of the absorbing medium. Irradiating a vapor of atomic mercury with a coherent field, with one component nearly resonant with the three-photon 6s S-1(0)-->6p P-1(1)o transition, and the other component the third harmonic of the first, we observe the interference between the two excitation pathways for this transition. Measurements of this interaction as the density of the mercury vapor is increased show evidence of nonlinear coupling (phase and amplitude locking) of the two field components with each other. This is manifested as a decrease in the magnitude of the photoionization signal, as well as a decrease of the depth of modulation of the photoionization signal.