About a decade before the first X-ray FELs scientists had already begun learning how to produce and characterize attosecond pulses of light using the interaction of strong infrared radiation with atoms in a process known as high-harmonic generation. Those exquisite bursts of light gave birth to the field of attosecond science, providing a powerful tool to explore and control the quantum mechanical motion of electrons and their interactions on their natural timescales²; indeed, the Nobel Prize in Physics 2023 was awarded to Pierre Agostini, Ferenc Krausz and Anne L’Huillier for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter. The desire to extend the wavelength and bandwidth of the light, to access inner shell dynamics, and to penetrate deeper into the materials all drove the fervent developments towards the ultraviolet and soft X-ray regime, despite the rapidly decreasing conversion efficiency.

More recently, advances in accelerator technology have enabled FELs to generate the large coherent bandwidth required to break the femtosecond barrier, producing much brighter attosecond pulses at short wavelengths. At the Linac Coherent Light Source (LCLS) in California, USA, teams have reported on attosecond pulse generation both in the soft (nanometre wavelength) and hard X-ray (Angstrom wavelength) regimes^3,4. Although the soft X-ray attosecond pulses saw immediate applications in spectroscopy experiments^5,6, the adaptation of hard X-ray attosecond pulses proceeded more slowly, in part because of a combination of the intrinsically smaller cross sections at shorter wavelengths, and the limited pulse energy.

在第一代 X 射线 FELs 发明前大约十年，科学家们就已经开始学习如何利用强红外辐射与原子的相互作用来产生和表征阿秒光脉冲，这个过程称为高谐波产生。这些精妙的光芒催生了阿秒科学领域，为探索和控制电子的量子力学运动及其在自然时间尺度上的相互作用提供了强大的工具²;事实上，2023 年诺贝尔物理学奖授予了 Pierre Agostini、Ferenc Krausz 和 Anne L'Huillier，以表彰他们在研究物质中电子动力学时产生阿秒光脉冲的实验方法。尽管转换效率迅速降低，但扩展光的波长和带宽、获得内壳动力学以及更深入地渗透到材料的愿望都推动了紫外和软 X 射线领域的热烈发展。

最近，加速器技术的进步使 FEL 能够产生打破飞秒障碍所需的大相干带宽，从而在短波长下产生更亮的阿秒脉冲。在美国加利福尼亚州的直线加速器相干光源 （LCLS） 中，团队报告了软 X 射线（纳米波长）和硬 X 射线（埃波长）范围内的阿秒脉冲产生^3,4。尽管软 X 射线阿秒脉冲立即应用于光谱学实验^5,6，但硬 X 射线阿秒脉冲的适应进展更慢，部分原因是较短波长下固有较小的横截面和有限的脉冲能量的结合。