The spectrum evolution of the femtosecond laser pulse in a three-level organic molecular medium (4, 4'-bis(dimethylamino) stilbene) is studied theoretically by solving the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain method. In the single-photon resonance case, for large area input pulses, the spectral components oscillating around ω32 appears in the spectrum due to the strong secondary excitation making the two-level model invalidity and the multi-level model should be used to describe this molecule. In the two-photon resonance case, part of the input pulse energy is transformed into higher and lower frequency components through amplified spontaneous emission and four-wave mixing, and the medium shows strong optical power limiting effect.%通过采用预估校正的时域有限差分法求解麦克斯韦-布洛赫方程,我们研究了飞秒激光脉冲在三能级有机分子(4, 4'-二甲氨基二苯乙烯分子)介质中传播时脉冲的频谱演化情况. 在单光子共振情况下,即入射脉冲频率等于1、2能级之间的共振频率,对大面积入射脉冲,由于强的二次激发的作用,电场频谱中出现了在ω32附近振荡的频率成分,ω32是2、3能级之间的共振频率,说明对大面积入射脉冲二能级模型已经失效,需要采用多能级模型来描述分子介质. 在双光子共振情况下,即入射脉冲频率等于1、3能级之间的共振频率的一半,由于介质中放大的自发辐射和四波混频的作用,部分入射脉冲能量转化为高频和低频电场成分的能量,分子介质表现出了很强的光功率限幅特性.
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