We investigate effect of atomic densities (N) on propagation and spectral property of femtosecond Gaussian pulses in a three-level Λ-type atomic medium by using numerical solution of full Maxwell-Bloch equations.It is shown that,for pulses with smaller area 2π,propagate in mediums with different N,pulse splitting does not occur and strength of spectral component near central frequency decreases considerably as N increases.For pulses with area 4π,pulse splitting could occur when pulses propagate in dilute medium with greater atomic density and in dense medium,and pulse spectral bandwidth and strength of spectral component with higher frequency increase with increasing of N in dilute medium.Spectrum broadening in dense medium is much smaller than that in dilute medium with greater atomic density.For pulses with larger area 8π,pulse splitting is similar to that of 4π pulses.Pulse spectral bandwidth and strength of spectral component with higher frequency increase monotonously with increasing of N.And spectrum broadening in dense medium is much larger than that in dilute medium with smaller atomic density.

With numerical solution of full Maxwell-Bloch equations obtained by finite-difference time-domain method and iterative predictor-corrector method,spatial distributions of pulse and populations of few-cycle laser pulse propagates in a ladder-type three-level atomic medium are investigated.It shows that,spatial distributions changes evidently with pulse area.As the pulse area is smallr,the pulse shape is irregular and oscillation time increases.As the pulse area is large,the pulse shape becomes relatively regular and oscillation times decreases evidently.With increasing of pulse area,amplitude and group velocity of pulse increase progressively,oscillation times of populations increase gradually and variation of spatial distributions of the pulse and populations at different moments decreases considerably.Moreover,spatial distribution of populations correlates closely to spatial distribution of the pulse.

Full Maxwell-Bloch equations are solved with PC-FDTD method to investigate population evolution of dense V-type three-level medium in few-cycle laser pulses.It is shown that the ratio of transition dipole moments(γ) has strong influence on appearing time and oscillation frequency of population inversion.As γ=1 population inversion with LFC appears later than that without LFC and near dipole-dipole(NDD) interaction delays population inversion.As γ>1,appearing with LFC is earlier than that without LFC and NDD interaction accelerates the population inversion.The number of population inversion with unequal transition dipole moments is more than that with equal transition dipole moments.Moreover,evolution of populations in a dense medium is quite different from that in a dilute medium.At input surface,in a dilute medium,population inversion appears with time evolution.With increasing of medium density,populations show quasi-periodicity oscillation and oscillation frequencies of populations with LFC are less than that without LFC.