Fan Tingting, Grychtol Patrik, Knut Ronny, Hernández-García Carlos, Hickstein Daniel D, Zusin Dmitriy, Gentry Christian, Dollar Franklin J, Mancuso Christopher A, Hogle Craig W, Kfir Ofer, Legut Dominik, Carva Karel, Ellis Jennifer L, Dorney Kevin M, Chen Cong, Shpyrko Oleg G, Fullerton Eric E, Cohen Oren, Oppeneer Peter M, Milošević Dejan B, Becker Andreas, Jaroń-Becker Agnieszka A, Popmintchev Tenio, Murnane Margaret M, Kapteyn Henry C
Department of Physics and JILA, University of Colorado, Boulder, CO 80309-0440;
Department of Physics and JILA, University of Colorado, Boulder, CO 80309-0440; Grupo de Investigación en Óptica Extrema, Universidad de Salamanca, Salamanca 37008, Spain;
Proc Natl Acad Sci U S A. 2015 Nov 17;112(46):14206-11. doi: 10.1073/pnas.1519666112. Epub 2015 Nov 3.
We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 µm, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N4,5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.
据我们所知，我们在电磁光谱的软X射线区域展示了首个明亮的圆偏振高次谐波光束，并利用它们在桌面规模装置中进行X射线磁圆二色性测量。使用波长分别为1.3微米和0.79微米的反向旋转圆偏振激光场，我们产生了光子能量超过160电子伏特的圆偏振谐波。谐波光谱呈现为一系列紧密间隔的左旋和右旋圆偏振峰对，其能量由能量守恒和自旋角动量决定。我们通过确定主导电子量子轨迹和最佳相位匹配条件来解释单原子和宏观物理现象。首次针对圆偏振谐波的先进相位匹配传播模拟揭示了有限相位匹配时间窗口对光谱的影响，以及独特的偏振形状阿秒脉冲序列。最后，据我们所知，我们在钆的N4、5吸收边进行了首次桌面X射线磁圆二色性测量，以验证该光源的高圆度、亮度和稳定性。这些结果证明了通过操纵驱动激光波形来操纵软X射线区域高次谐波的偏振、光谱和时间形状的可行性。
