用太赫兹辐射观测和操控强场动力学

Intense terahertz field has become a new tool for the manipulation of quantum states in strong field physics, which has been demonstrated by the generation of high-order sidebands by electron-hole collisions of excitons in the semiconductor and high-order harmonics by Bloch oscillations in the crystal. The intense terahertz field with single-cycle, stabilized carrier-envelope phase and controlled polarization can be generated by the ionization of gas in the strong femtosecond laser field. This intense terahertz field can be used to optimize the orientation of molecules, enhance the efficiency of high-harmonic generation and modulate the momentum distribution of photoelectrons. This proposal will measure simultaneously terahertz radiation, high-order harmonics and above-threshold ionization of atoms and oriented molecules, which are tunnel-ionized with varying the relative phase, intensity, polarization and wavelength of the two-color laser field. The Coulomb effect on the terahertz radiation will be well studied and the spatial distribution of terahertz radiation in the near field will be carefully characterized. It emphasizes the manipulation of the dynamics of electronic wavepackets in the strong laser field using intense terahertz field. The achievement of this project will increase the efficiency of terahertz generation, develop the mew method to diagnose the phase of a laser filed in situ and to map molecular potentials, even realize the muti-time-scale manipulation of the complex evolution of quantum states using whole electromagnetic spectra covered from terahertz to X-ray.

半导体中电子-空穴碰撞产生的高阶太赫兹边带以及太赫兹脉冲在晶体中产生的高次谐波都表明强太赫兹场已经成为强场物理中一种新的量子操控手段。飞秒激光脉冲电离气体产生的单周期、载波包络相位稳定、偏振态可控的强太赫兹场，可以优化分子的取向、提高高次谐波的辐射效率、调制光电子的动量分布。本项目通过控制双色激光场的相位、光强、偏振、波长，同时测量原子和定向分子隧穿电离产生的太赫兹谱、高次谐波谱和光电子动量谱，实验研究库仑场在太赫兹辐射中的作用，刻画太赫兹辐射的近场空间分布，并用产生的强太赫兹场调制电子波包的动力学过程。项目结果有助于提高太赫兹辐射的效率，发展用太赫兹辐射去原位测量激光相位的技术，用太赫兹辐射对分子势成像的方法，实现从太赫兹波到X射线，对复杂量子态演化的多时间尺度的观测和全电磁波段的操控。

超短强激光脉冲电离气体，能够产生具有重要应用价值的从太赫兹到软X射线波段的超宽带相干辐射。本项目所建立的基于光场调控的高次谐波和太赫兹波光谱（HATS）同步测量和调控技术，为更全面、深刻地揭示强场驱动下原子、分子内部的电子超快动力学和辐射过程提供了新手段。研究发现，太赫兹辐射强度可以标校原子分子在强场中的电离率，太赫兹辐射的最优相位可以用作强场物理中中原位的相位计。我们将HATS技术用于准直双原子分子，建立了与电离模型无关的自由空间分子角分辨的微分电离截面全光测量新手段，成功揭示了N2分子高次谐波产生过程中的双中心干涉效应。将HATS技术拓展至二维（2D-HATS），通过精细调整激光场的偏振和双色场的相位，精准控制CO2分子的准直角度和电子波包的轨迹，揭示了CO2分子高阶谐波产生过程中的多通道相干效应，实现原位阿秒脉冲整形。