Femtosecond pulse propagation in medium of two-level atoms

The interaction of femtosecond pulse with an extended medium of the two-level atoms or molecules is considered. It is shown that in the ultrahigh intensity laser field the atomic response is extremely nonmonochromatic due to the nonlinearity appropriate to the two-level system. The theoretical analyses shows that the two-level system tends to the generation of harmonics of the driving field. The results of the computer simulations show that in the ultrahigh fields the spectrum of the medium response is unusually broad in comparison with that for the fields of moderate intensity.