Coherent controll of electronic trajectory and generation of short-wavelength attosecond pulses in a shaped intense laser field

自从激光发明以来，利用超短的分辨能力探测自然界中的动力学过程极大地依赖于超短脉冲。正因为需要更加精细的时间分辨和更高的峰值功率，研究人员一直致力于更短脉宽的激光脉冲研究。目前在拍瓦（1015 W）量级激光器上获得的最高聚焦强度已超过1021 W/cm2，而超短激光脉冲时间宽度则已短至几个飞秒（10-15 s）。现在利用超短超强激光与固体、气体、以及等离子体等相互作用产生X射线的研究已经成为强场激光物理领域的热点，并推动了激光泵浦X射线源在生物成像和原子、分子内部超快动力学探测等方面的应用。 高次谐波产生过程提供了一种简单的傅里叶合成阿秒脉冲的方法。将一束激光聚焦至原子或者分子气体，可以产生具有非常高级次谐波相干辐射。随着各个国家的研究人员在阿秒物理上的不断探索，2008年，在实验上已经获得了脉宽为80阿秒的单个阿秒脉冲。然而，这样的阿秒脉冲相干光源的获得，仍需要非常苛刻的条件，比如载波包络相位稳定的周期量级激光系统，超宽带极紫外波段反射镜等，这些条件都限制了短波长阿秒脉冲光源的产生和应用。 本论文绕过产生单个阿秒脉冲必须的苛刻条件，结合现有的实验条件，在理论上重点研究了利用中红外波段的整形双色强场对高次谐波过程中的电子轨道进行相干控制，来获得脉宽突破50阿秒的水窗波段单个阿秒脉冲。论文主要研究结果和创新成果包括以下几个方面： 1. 基于Lewenstein模型研究了多周期量级双色中红外光场（波长为2000-nm）驱动氦原子产生高次谐波辐射过程。从理论上验证了在适当地调节两光场间相对相位后，可以获得水窗波段的脉宽为67阿秒的单个阿秒脉冲。与只有单色基频场驱动下的情况相比较，这一双色场机制可以使XUV波段的辐射强度增加一个数量级左右，这对于中红外光场中高能量的谐波产生非常有意义。 2. 研究了带适量正啁啾的周期量级基频光场（中心波长900-nm）和亚谐波光场（中心波长1800-nm）组成的整形强双色场驱动氦原子产生高次谐波过程中电子轨道的相干控制，可以对水窗波段的谐波超连续谱固有啁啾进行相位补偿，获得脉宽（69阿秒）接近傅里叶极限脉宽（53阿秒）水窗波段（带宽69eV，中心波长为3.67nm）的单个阿秒脉冲。 3. 我们提出了一种通过利用多周期量级、啁啾可控的整形双色场产生高次谐波来获得单个阿秒脉冲的新方案。与之前产生单个阿秒脉冲所必须要求的驱动光载波包络相位（CEP）稳定所不同，在单原子响应范畴，单个阿秒脉冲的稳定产生不依赖驱动光场的CEP。由于此方案对于激光参数波动不敏感，有利于在实验上实施。 4. 在实验上参与提出了一种新的方案，在一束强的基频激光场中加入另一束弱的谐波光场来驱动产生高次谐波，通过简单地调节两光场之间的相对延时，可以对谐波进行动态啁啾补偿和脉冲压缩，产生脉宽接近傅里叶极限脉宽的阿秒脉冲链。The measurement of ultrashort time intervals and the perception of the dynamics of nature are highly dependent on light pulses. The need of finer time resolution and the quest for higher peak power explain the continuous trend towards shorter laser pulses almost since the inception of the laser. Now petawatt (PW) and terawatt (TW) lasers are applied widely, and the highest intensity is over 10^21 W/cm2, while the shortest pulse duration is down to several femtoseconds and few cycles. These achievements supply a powerful support for the generation of nanometer and sub-nanometer wavelength X-ray, especially the coherent X-ray, which is currently being a hot subject in the high intensity laser physics for its wide applications in biologic imaging and ultrafast dynamics detecting, etc. High-order harmonic generation (HHG) seems to offer an easier method for producing attosecond pulses by Fourier synthesis. An intense infrared laser pulse focused in an atomic or molecular gas induces coherent emission of odd-order harmonics with very high orders. Continuing efforts in attosecond physics by the scientists in the wide world have led to an isolated attosecond pulse (IAP) with the duration of 80 as experimentally in 2008. However, owing to the limitation to the stabilized carrier-envelop phase (CEP) laser system and the ultrawide bandwidth of the mirror, the generation and the application of IAPs is an extremely difficult task in the experiments. Beyond the obstacles mentioned above, the main researches in this dissertation include: Theoretical and experimental study on coherently controlling of electronic trajectory by shaped intense laser field combined by the two-color pulses in the mid-infrared regime, we can obtaine IAPs with the duration of sub-50-as. The main results and innovative results of the thesis are listed as follows: 1. Based on the Lewenstein model, we investigate high-order harmonic generation in helium atoms driven by a multi-cycle two-colour optical field synthesized by an intense 2000 nm, 20 fs pulse and its frequency-doubled pulse. When the latter is slightly detuned and properly phase shifted with respect to the fundamental laser pulse，a supercontinuum in the water window can be selected to yield an isolated 67 attosecond pulse. In addition, this two-colour scheme can enhance the XUV emission intensity by one order of magnitude, which is important for HHG in an IR field. 2. A theoretical investigation is presented that uses a strong two-color laser field composed of a linearly chirped fundamental (900-nm) and its subharmonic (1800-nm) laser pulses to control coherently the electronic quantum path in HHG. As a result, the phase compensation of harmonic emission can be achieved, as well as the intrinsic chirp can be reduced significantly. We have also demonstrated that an intense 69-as XUV IAP (with a bandwidth of 69 eV at a central wavelength of 3.67 nm) can be obtained directly. 3. We propose a new scheme for generating isolated attosecond pulses via high-order harmonic generation in gases using a chirp-controlled two-color laser field of multi-cycle duration. In contrast to previous techniques where the stable CEP of the driving laser pulses is vital for isolated attosecond pulse generation, the generation of isolated attosecond pulses based on the proposed scheme is robust against the variation of CEP in the single-atom response regime. The scheme is promising for producing isolated attosecond pulses in the water window region and robust against laser parameter fluctuations. 4. We propose a new scheme for the chirp compensation and pulse compression in HHG by adding a weak second harmonic laser field to the fundamental laser field for HHG. A nearly transform-limited attosecond pulse train has been produced by using the two-color laser pulses with an appropriate relative timedelay. This technique can also be used for the generation of strong transform-limited isolated attosecond pulses