Rabi frequency and nonlinearity of a two-level atom for an ultrashort optical pulse

We investigate the interaction of a two-level atom with a short femtosecond optical pulse without the rotating-wave approximation. We have derived the population inversion due to the counterrotating terms. It is shown that due to the counterrotating terms the population inversion oscillates with high frequency and there is the effective time-dependent addition to the Rabi frequency that varies inversely proportionally to the length of an optical pulse and this effect is actual for a short femtosecond optical pulse. It is shown that there is the transient cubic nonlinearity due to the counterrotating terms.