Self-action nonlinearity is a key aspect – either as a foundational element or a detrimental factor – of several optical spectroscopies and photonic devices. Supercontinuum generation, wavelength converters, and chirped pulse amplification are just a few examples. The recent advent of Free Electron Lasers (FEL) fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet (EUV) and X-ray regimes. No evidence for intrapulse dynamics, however, has been reported at such short wavelengths, where the light-matter interactions are ruled by the sharp absorption edges of core electrons. Here, we provide experimental evidence for self-phase modulation of femtosecond FEL pulses, which we exploit for fine self-driven spectral tunability by interaction with sub-micrometric foils of selected monoatomic materials. Moving the pulse wavelength across the absorption edge, the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening. These findings are rationalized accounting for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold, respectively.

自作用非线性是几个光学光谱学和光子器件的关键方面，无论是作为基础元素还是有害因素。超连续谱生成，波长转换器和chi脉冲放大仅是几个示例。自由电子激光器（FEL）的最新出现促进了非线性的发展，以提出新的概念并扩展了用于极端紫外线（EUV）和X射线方案的光波长范式。然而，在如此短的波长下，没有报道过脉冲内动力学的证据，在这种短波长下，光-物质相互作用是由核心电子的尖锐吸收边缘决定的。在这里，我们提供了飞秒FEL脉冲自相位调制的实验证据，我们通过与选定的单原子材料的亚微米级箔片相互作用来开发精细的自驱动光谱可调性。在吸收边缘上移动脉冲波长，光谱轮廓从非线性光谱蓝移变为红移加宽。这些发现是合理的，分别解释了在阈值之上和之下的高激发电子的超快电离和延迟的热响应。