To shorten the duration of x-ray pulses, we present a nonlinear optical technique using atoms with core-hole vacancies (core-hole atoms) generated by inner-shell photoionization. The weak Coulomb screening in the core-hole atoms results in decreased absorption at photon energies immediately above the absorption edge. By employing this phenomenon, referred to as saturable absorption, we successfully reduce the duration of x-ray free-electron laser pulses (photon energy: 9.000 keV, duration: 6–7 fs, fluence: $2.0–3.5\times {10}^5\mathrm{J}/{\mathrm{cm}}^2$) by $\sim 35\%$. This finding that core-hole atoms are applicable to nonlinear x-ray optics is an essential stepping stone for extending nonlinear technologies commonplace at optical wavelengths to the hard x-ray region.

中文翻译：

---

通过饱和吸收缩短 X 射线脉冲持续时间

为了缩短 X 射线脉冲的持续时间，我们提出了一种非线性光学技术，该技术使用由内壳光电离产生的具有核孔空位（核孔原子）的原子。核孔原子中的弱库仑屏蔽导致吸收边缘正上方光子能量的吸收减少。通过利用这种称为饱和吸收的现象，我们成功地减少了 X 射线自由电子激光脉冲的持续时间（光子能量：9.000 keV，持续时间：6-7 fs，注量：$2.0\mathrm{——}3.5\times {10}^5\mathrm{J}/{\mathrm{厘米}}^2$） 经过 $～35\%$. 核孔原子适用于非线性 x 射线光学的这一发现是将光学波长常见的非线性技术扩展到硬 x 射线区域的重要垫脚石。