Ultrafast phenomena on a femtosecond timescale are commonly examined by pump-probe experiments. This implies multiple measurements, where the sample under investigation is pumped with a short light pulse and then probed with a second pulse at various time delays to follow its dynamics. Recently, the principle of streaking extreme ultraviolet (XUV) pulses in the temporal domain has enabled recording the dynamics of a system within a single pulse. However, separate pump-probe experiments at different absorption edges still lack a unified timing, when comparing the dynamics in complex systems. Here, we report on an experiment using a dedicated optical element and the two-color emission of the FERMI XUV free-electron laser to follow the charge and spin dynamics in composite materials at two distinct absorption edges, simultaneously. The sample, consisting of ferromagnetic Fe and Ni layers, separated by a Cu layer, is pumped by an infrared laser and probed by a two-color XUV pulse with photon energies tuned to the M-shell resonances of these two transition metals. The experimental geometry intrinsically avoids any timing uncertainty between the two elements and unambiguously reveals an approximately 100 fs delay of the magnetic response with respect to the electronic excitation for both Fe and Ni. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. We furthermore observe that the electronic dynamics of Ni and Fe show pronounced differences when probed at their resonance, while the demagnetization dynamics are similar. These observations underline the importance of simultaneous investigation of the temporal response of both charge and spin in multi-component materials. In a more general scenario, the experimental approach can be extended to continuous energy ranges, promising the development of jitter-free transient absorption spectroscopy in the XUV and soft X-ray regimes.

中文翻译：

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Fe-Cu-Ni三层中超快充电和自旋动力学的同时双色快照视图

飞秒时间尺度上的超快现象通常通过泵浦探针实验来检查。这意味着需要进行多次测量，在此过程中，将被测样品用短光脉冲泵浦，然后在不同的时间延迟下用第二个脉冲探测其动态。近来，在时域中条纹化极紫外（XUV）脉冲的原理使得能够在单个脉冲内记录系统的动态。但是，当比较复杂系统中的动力学时，在不同吸收边缘的单独泵浦探针实验仍然缺乏统一的时间安排。在这里，我们报告了一个实验，该实验使用专用光学元件和FERMI XUV自由电子激光器的双色发射，以同时跟踪复合材料在两个不同的吸收边缘处的电荷和自旋动力学。例子，由铁磁性的Fe和Ni层组成，并由Cu层隔开，由红外激光泵浦，并由双色XUV脉冲探测，该双色XUV脉冲的光子能量已调谐到这两种过渡金属的M壳共振。实验几何学本质上避免了两个元素之间的任何时序不确定性，并且明确地揭示了相对于Fe和Ni的电子激发，磁响应的延迟约为100 fs。该延迟表明，在退磁过程中，电子自由度和自旋自由度被解耦。我们进一步观察到，当镍和铁的共振时，它们的电子动力学表现出明显的差异，而去磁动力学是相似的。这些观察结果强调了同时研究多组分材料中电荷和自旋的时间响应的重要性。在更一般的情况下，实验方法可以扩展到连续能量范围，从而有望在XUV和软X射线方案中发展无抖动的瞬态吸收光谱。