Nonlinear light–matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUV-probe spectroscopy of core electrons. Such interactions are now routinely investigated at free-electron laser (FEL) facilities. Yet, electron dynamics are often too fast to be captured with the femtosecond resolution of state-of-the-art FELs. Attosecond pulses from laser-driven XUV-sources offer the necessary temporal resolution. However, intense attosecond pulses supporting nonlinear processes have only been available for photon energy below 50 eV, precluding XUV-pump/XUV-probe investigation of typical inner-shell processes. Here, we surpass this limitation by demonstrating two-photon absorption from inner electronic shells of xenon at photon energies around 93 eV and 115 eV. This advance opens the door for attosecond real-time observation of nonlinear electron dynamics deep inside atoms.

中文翻译：

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100 eV光谱区域中的台式非线性光学器件

极紫外（XUV）中的非线性光-物质相互作用是执行核心电子的XUV-泵浦/ XUV-探针光谱的先决条件。现在，通常在自由电子激光（FEL）设施中对此类相互作用进行研究。然而，电子动力学通常太快而无法用最新FEL的飞秒分辨率捕获。来自激光驱动的XUV源的阿秒脉冲提供了必要的时间分辨率。但是，支持非线性过程的强阿秒脉冲仅适用于低于50 eV的光子能量，这排除了对典型内壳过程的XUV泵/ XUV探针研究的可能性。在这里，我们通过证明氙的内部电子壳在93 eV和115 eV附近的光子能量处的双光子吸收来超越此限制。