Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
Center for Free-Electron Laser Science CFEL, Deutsches-Elektronen Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany.
Phys Rev E. 2023 Jan;107(1-2):015205. doi: 10.1103/PhysRevE.107.015205.
Saturable absorption is a nonlinear effect where a material's ability to absorb light is frustrated due to a high influx of photons and the creation of electron vacancies. Experimentally induced saturable absorption in copper revealed a reduction in the temporal duration of transmitted x-ray laser pulses, but a detailed account of changes in opacity and emergence of resonances is still missing. In this computational work, we employ nonlocal thermodynamic equilibrium plasma simulations to study the interaction of femtosecond x rays and copper. Following the onset of frustrated absorption, we find that a K-M resonant transition occurring at highly charged states turns copper opaque again. The changes in absorption generate a transient transparent window responsible for the shortened transmission signal. We also propose using fluorescence induced by the incident beam as an alternative source to achieve shorter x-ray pulses. Intense femtosecond x rays are valuable to probe the structure and dynamics of biological samples or to reach extreme states of matter. Shortened pulses could be relevant for emerging imaging techniques.
饱和吸收是一种非线性效应，其中由于光子的大量涌入和电子空位的产生，材料吸收光的能力受到阻碍。在实验中诱导铜的饱和吸收会导致传输的 X 射线激光脉冲的持续时间缩短，但关于不透明度的变化和共振的出现的详细描述仍然缺失。在这项计算工作中，我们使用非局部热力学平衡等离子体模拟来研究飞秒 X 射线与铜的相互作用。在受挫吸收开始后，我们发现发生在高电荷态的 K-M 共振跃迁使铜再次变得不透明。吸收的变化产生了一个短暂的透明窗口，负责缩短传输信号。我们还提出使用入射束诱导的荧光作为实现更短 X 射线脉冲的替代源。强飞秒 X 射线可用于探测生物样品的结构和动力学，或达到物质的极端状态。缩短的脉冲可能与新兴的成像技术有关。
