Growth of parametric fields in (2+1)-photon laser ionization of atomic oxygen

The multiphoton excitation of an atom by a strong pulsed laser field creates a transient inversion of the pumped, excited level with respect to a lower-lying intermediate level. This results in a coupled time-and-distance traveling-wave situation appropriate for the growth of auxiliary radiation fields that are coupled to the excited, intermediate, and ground levels. Under certain circumstances these fields can suppress the multiphoton excitation process. The two-photon pumping of a ground ${}_{}{}^3{}_{}{}^{}$P level of O to the first excited ${}_{}{}^3{}_{}{}^{}$P level and ionization to the continuum by the same field are analyzed for the growth of amplified spontaneous emission and parametric four-wave mixing. A threshold growth condition is used to interpret the numerical solutions of the appropriate Maxwell-Bloch equations. A novel transition from exponential to linear growth of the parametric waves is observed in the calculations.