超短脉冲laser的时间包络对$e^+e^-$对纵动量谱的影响
Imprint of temporal envelope of ultrashort laser pulses on longitudinal momentum spectrum of $e^+e^-$ pairs
摘要：
利用量子动力学方程研究了强脉冲的时间脉冲形状对$e^+e^-$对动量分布的影响。两个非常相似的时间包络，即高斯和Sauter，保持所有其他脉冲参数相同，被认为是为此目的。相反的共同的看法，这可以衡量从可互换使用这些时间配置文件，由两个脉冲创建的对的纵向动量谱被发现在所有的时间制度显着不同。对于具有几个周期的振荡的脉冲，脉冲的时间分布揭示在振荡干涉图案骑在否则平滑的纵向动量谱在渐近时间。由于具有多个势垒的时间结构的脉冲的散射势的反射振幅的量子干涉引起的振荡的发生对于高斯脉冲比对于Sauter脉冲的周期数振荡。此外，对于高斯脉冲，在脉冲持续时间内相同数量的振荡的振荡幅度更大。载波包络相位和频率啁啾的存在被发现放大这些差异。在没有任何明显的干扰效应的脉冲具有少于五个振荡，纵向动量谱有一个更高的峰值为Sauter脉冲在渐近时间。而在演化的过渡阶段之前，谱的峰值则表现出相反的趋势。
英文摘要：
The effect of the temporal pulse shape of intense pulses on the momentum distribution of $e^+e^-$ pairs is studied using the quantum kinetic equation. Two closely resembling temporal envelopes namely, Gaussian and Sauter, keeping all the other pulse parameters the same, are considered to this end. Contrary to the common perception which can be gauged from the interchangeable use of these temporal profiles, the longitudinal momentum spectrum of the pairs created by the two pulses is found to differ significantly in all the temporal regimes. For the pulses having a few cycles of oscillations, the temporal profile of the pulse is revealed in the oscillatory interference pattern riding over the otherwise smooth longitudinal momentum spectrum at asymptotic times. The onset of the oscillation due to the quantum interference of reflection amplitudes from the scattering potential due to the pulses having a temporal structure of multiple barriers takes place for few-cycle oscillations for the Gaussian pulse than that for the Sauter pulse. Furthermore, the oscillation amplitude for the same number of oscillations within the pulse duration is larger for the Gaussian pulse. The presence of the carrier-envelope phase and the frequency chirping is found to magnify these differences. In the absence of any appreciable interference effect for the pulses having less than five oscillations, the longitudinal momentum spectrum has a higher peak value for the Sauter pulse at asymptotic times. On the hand, before the transient stage of evolution, the peak of the spectrum shows the opposite trend.
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