Relative carrier-envelope phase dependence of resonant propagation of two-colour femtosecond pulses in V-type atomic medium

Using numerical solution of the full Maxwell-Bloch equations, which is obtained by the finite-difference time-domain method and the iterative predictor-orrector method, we investigate the modulation effect of relative carrierenvelope phase （hereinafter referred to as the relative phase） on resonant propagation of two-colour femtosecond ultrashort laser pulses in a V-type three-level atomic medium. It is found that the pulse splitting occurs for a smaller value of relative phase; when the value of relative phase increases to a certain value, only the variation of pulse shape is present and the pulse splitting does not occur any more; moreover, when the value of relative phase is smaller, the pulse group velocity is larger. The relative phase also has an obvious effect on population and spectral property. Different population transfers can be realized by adjusting the value of relative phase. Generally speaking, for the pulses with smaller areas their spectral strengths and frequency ranges decrease obviously with the value of relative phase increasing; for the pulses with larger areas, with value of the relative phase increasing, their spectral strengths decrease remarkably but the relative strengths of the higher frequency components increase significantly, while the spectral frequency range is not varied evidently.

Carrier-envelope phase measurement using plasmonic-field-enhanced high-order harmonic generation of H atom in few-cycle laser pulses

We investigate the plasmonic-field-enhanced high-order harmonic generation（HHG） of H atom driven by few-cycle laser pulses, by solving the time-dependent Schr？dinger equation（TDSE）. Compared with the homogeneous field, HHG spectra generated by inhomogeneous field exhibit two-plateau structure. We analyze the origin of the two plateaus by using the semiclassical trajectory method. Our results from both classical and TDSE simulations show that the cutoffs of the two plateaus are dramatically affected by the carrier-envelope phase（CEP） of laser pulse in the inhomogeneous field, even for a little longer pulse. Thus, we can determine the CEP of driving laser based on the cutoff position of HHG generated in the inhomogeneous field.

A Scaling Law of Photoionization in Ultrashort Pulses

By means of the numerical solution of time-dependent Schr6dinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by ka times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.

静电场对V型三能级介质中传播的超短脉冲频谱CEP相关性的影响

利用由预估校正（PC）-时域有限差分（FDTD）法得到的Maxwell—Bloch方程的数值解研究了静电场对在V型三能级介质中传播的飞秒超短激光脉冲频谱载波包络相位（CEP）相关性的影响．结果表明：未加入静电场时，CEP的变化对脉冲频谱中较低频率部分的强度有较明显的影响，而对高频部分的影响很微弱，频谱平坦性和连续性较差；加入静电场后，CEP对频谱强度尤其是较高频部分强度的影响变大，通过选择适当的静电场和CEP的值可以获得平坦性和连续性较好的超连续谱．另外，静电场的加入还使得到相同频谱分布所对应的CEP周期由π改变为2π．

氢原子强场电离的经典轨迹

在经典理论模型的基础上,利用经典牛顿力学的MONTE-CARLO法,通过解牛顿方程,从理论上研究氢原子在激光场中的经典动力学轨迹。研究发现,线偏振激光场下,电子在脉冲作用下吸收能量在大约一个光学周期后发生电离,电子最终出射的方向受激光场脉冲的影响。当总的激光脉冲周期TD=4T(T为一个光学周期)时,电子向同一个方向出射;当TD=5T时,电子向两边出射;改变激光脉冲的载波包络相位(CEP),电子的出射轨迹也发生改变。

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses（IAP） are sensitive to the carrier-envelope phase（CEP）of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized（TCCRCP） laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.

椭圆偏振激光脉冲驱动的氙原子强场双电离对载波包络相位的依赖

利用经典系综模型,研究了椭圆偏振激光脉冲驱动的氙原子强场双电离。计算结果表明,Xe2+的末态动量分布强烈依赖于载波包络相位(CEP)。在该激光脉冲驱动下,双电离事件中同时存在次序双电离(SDI)和非次序双电离(NSDI)。SDI产率随CEP增大先减少后增大,NSDI产率随CEP增大先增大后减小,两者均呈周期变化,其周期为π。轨迹分析显示,NSDI事件仍然是由再碰撞而发生,并且该过程对应的物理机制能够由三步模型很好地解释;另外,SDI过程中两电子的电离时间以及NSDI过程中发生再碰撞的时间均强烈依赖于CEP,从而使得Xe2+的末态动量分布随CEP的改变而变化。

激光的组合场相对相位对CO分子产生阿秒脉冲的影响

利用强场近似方法研究了CO分子在组合场中发射高次谐波和阿秒脉冲的性质,组合场由波长为800nm和2000nm的两束激光形成。考虑了分子在组合场中的线性Stark位移对CO分子能级的影响,通过优化组合场的相对载波相位驱动分子。研究表明CO分子在这种组合场中不仅高次谐波的截止位置有了扩展,并且由于这种不对称分子在某一方向的电离抑制,减少了电子在演化过程的干涉效应,由此得到的高次谐波有很好的超连续性。叠加平台区域的高次谐波均能得到单个阿秒脉冲,脉冲宽度最窄可达到98as,与单色对称场驱动CO分子产生的阿秒脉冲相比较,其脉冲宽度有了明显的缩短。

基于高能光电子非对称性的少周期激光载波包络相位确定

提出了一种利用高能光电子的非对称角分布来确定少周期激光载波包络相位(CEP)的方法。通过二级强场近似方法研究了隧穿电离区域氦原子在少周期激光驱动下阈上电离高能光电子角分布对载波相位的依赖关系,选择合适能量范围的光电子分布,能够给出非对称参数随激光CEP的变化。研究结果表明,通过这种高能光电子得到的非对称参数和由数值求解含时薛定谔方程(TDSE)得到的非对称参数随CEP的变化趋势符合得很好。由数值求解TDSE获得的非对称参数包含了低能光电子的贡献,而二级强场近似的方法仅通过高能光电子得到,从而该方法为实验测定激光的载波包络相位提供了方便。

少周期飞秒激光场下Ne原子的非顺序双电离

本文实验测量了少周期飞秒激光场下惰性气体Ne原子非顺序双电离引起的Ne^(2+)离子动量分布,发现该分布强烈依赖于激光场的载波-包络相位(Carrier-Envelope Phase,CEP),在某些CEP条件下呈现出明显不对称的双峰结构.半经典理论计算可以定性地重现实验结果.理论分析表明,二价离子的不对称动量分布来自于非顺序双电离过程中碰撞直接电离(Recollision-Impact-Ionization,RII)与碰撞-激发电离(Recollision-Excitation and Subsequent Ionization,RESI)两个通道的贡献,其中不对称双峰结构来自于RII通道,而RESI通道影响的是低动量部分,导致双峰结构变平.RII通道中产生的Ne^(2+)离子动量分布对CEP有较强的依赖性,而RESI通道中Ne^(2+)离子动量分布随CEP的变化不明显.进一步的计算表明,离子实库仑势在非顺序双电离过程中起到非常重要的作用,它将引起RESI通道产量增大.