中文摘要：

利用由预估校正（PC）-时域有限差分（FDTD）法求得的不含慢变包络近似（SVEA）和旋转波近似（RWA）的全波Maxwell-Bloch方程的数值解,研究了飞秒啁啾Gauss型激光脉冲（以下简称啁啾脉冲）在稠密Λ型三能级原子介质中的传播.研究表明,啁啾系数（C）的正负及大小的变化对脉冲传播特性有显著的影响,而且这个影响与脉冲面积的大小密切相关.面积小于4π的啁啾脉冲,在介质中传播时不发生分裂,且啁啾脉冲逐渐演化为一个近似的无啁啾（C=0）脉冲,这一特点不随啁啾系数的改变而改变,但啁啾系数的变化将改变脉冲的振幅和群速度.正啁啾（C〉0）情况,振幅和群速度随C的增大而减小;负啁啾（C〈0）情况,振幅和群速度则不随C的绝对值的增大而单调地增加或减小.面积等于和大于4π的啁啾脉冲,在介质中传播时都将分裂成不同数目和形状的子脉冲,脉冲分裂的早晚和数目决定于啁啾系数的正负及大小.但是,两种情况下脉冲分裂方式大不相同,啁啾系数影响也不同.脉冲面积等于4π时,大啁啾系数导致子脉冲数目增加,而脉冲面积大于4π时,大啁啾系数导致子脉冲数目减少.不管脉冲面积大小,啁啾系数正负及大小的变化都对介质粒子数布居产生明显的影响。

英文摘要：

We investigate propagation of femtosecond chirped Gaussian laser pulse in a dense three-level Λ-type atomic medium by using the numerical solution of the full Maxwell-Bloch equations without the slowly varying envelope and the rotatingwave approximations,and the solution is obtained by PC-FDTD method.It is shown that,variation of the sign and size of the chirp coefficient has considerable effect on pulse propagation property,and the effect is closely relative to size of the pulse area.When the area of chirped pulse is smaller than 4π,splitting doesn＇t occur and the chirped pulse evolves gradually to an approximate normal Gaussian pulse（C = 0）,and this characteristic doesn＇t vary with the chirp coefficient varying.However,variation of the chirp coefficient will changed the amplitude and group velocity of the pulse.For the positive chirp（C 0）,amplitude and group velocity of the pulse decrease with chirp coefficient increasing;for the negative chirp（C 0）,amplitude and group velocity of the pulse don＇t monotonically increase or decrease with absolute value of C increasing.Both the chirped pulses with area equal to larger than 4π will split into sub-pulses of different numbers and shapes,the time and number of the pulse splitting will be determined by the sign and size of the chirp coefficient.But in the two cases,the pulse splitting patterns are much different,and the effects of the coefficient are also different.When the pulse area equals 4π,larger chirp coefficient will lead to increased sub-pulse number,but when the pulse area is larger than 4π,larger chirp coefficient will lead to decreased sub-pulse number.In addition,regardless of pulse area being larger or smaller,changing sign and size of the chirp coefficient always produces obvious effect on the atomic population.