Tunable x-ray radiation sources are of wide importance for imaging and spectroscopy in fundamental science, medicine, and industry. The growing demand for highly tunable, high-brightness lab-scale x-ray sources motivates research of new mechanisms of x-ray generation. Parametric x-ray radiation (PXR) is a mechanism for tunable x-ray radiation from free electrons traversing crystalline materials. Although PXR has been investigated over decades, it remained limited in usages due to the low flux and strict dependence on fixed crystal properties. Here, we find new effects hiding in the PXR mechanisms, which provide control over the radiation polarization and spatial and temporal distribution. The radiation can form ultrashort pulses and delta-pulse trains, which makes the new effects fundamentally different from all conventional mechanisms of x-ray generation. We show how these new effects can be created from free-electron interactions with van der Waals materials. Furthermore, we consider free electrons traversing near material edges, which provides an additional degree of tunability in angular distribution and polarization of PXR. Our findings enable us to utilize recent breakthroughs in the atomic-scale design of 2D material heterostructures to provide platforms for creating tunable x-ray pulses.

中文翻译：

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基于自由电子晶体相互作用的 X 射线脉冲的时空设计

可调谐 X 射线辐射源对于基础科学、医学和工业中的成像和光谱学具有广泛的重要性。对高度可调、高亮度的实验室级 X 射线源的需求不断增长，激发了对 X 射线产生新机制的研究。参数 X 射线辐射 (PXR) 是一种可调谐 X 射线辐射的机制，该机制来自自由电子穿过晶体材料。尽管 PXR 已经被研究了几十年，但由于低通量和对固定晶体特性的严格依赖，它的使用仍然受到限制。在这里，我们发现隐藏在 PXR 机制中的新效应，它提供了对辐射极化和空间和时间分布的控制。辐射可以形成超短脉冲和增量脉冲串，这使得新效应与所有传统的 X 射线产生机制完全不同。我们展示了如何通过与范德华材料的自由电子相互作用来产生这些新效应。此外，我们考虑了自由电子在材料边缘附近穿越，这为 PXR 的角度分布和极化提供了额外的可调性。我们的发现使我们能够利用最近在 2D 材料异质结构的原子级设计方面取得的突破，为创建可调谐 X 射线脉冲提供平台。