Evolution of maximal coherence and nonlinear mixing in an ionizing three-level medium

Abstract

We have carried out a numerical investigation of the influence of photoionization, causing neutral atom depletion and accompanying electron impact dephasing, on a stimulated Raman adiabatic passage (STIRAP) scheme in a dense medium of three-level atoms. The nonlinear mixing from the large (maximal) coherence generated during the population transfer is calculated within the framework of a time-dependent model for a single atom with simplified propagation appropriate to a thin slab of material. For definiteness and consistency of the atomic parameters employed we have treated a scheme in atomic calcium. For generation of intense vacuum ultraviolet fields, laser intensities in the range from 100 ${\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ to 1 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ are required, and these are found to cause significant ionization of the atom. We find that at atomic densities $g{10}^{16}/{\mathrm{cm}}^{3}$ the effects of electron impact dephasing on the coherence evolution is very significant and will dominate over other dephasing mechanisms. In contrast at lower densities $\ensuremath{\sim}{10}^{14}/{\mathrm{cm}}^{3}$ (compatible with a sealed vapor cell) the conversion efficiency is relatively unaffected by electron impact dephasing. We conclude that in general photoionization depletion and electron impact dephasing will be significant in maximal coherence wave mixing and STIRAP population transfer schemes operated at high density.