Time-resolved angle-resolved photoemission spectroscopy is one of the most powerful pump–probe measurements of materials driven far from equilibrium. Unlike the linear-response regime, where the frequency-dependent response function is independent of time, in a far-from-equilibrium experiment, the response function depends on two times in the time domain. In this work, we describe how one can use time-dependent frequency response functions and how they involve contributions from times that are near to each other. This implies that they should not be thought of as a frequency-dependent response at a single definite time. Instead, the Fourier uncertainty relations show that they involve contributions from ranges of times and must be interpreted in this light. We use this insight to help understand what time-resolved photoemission measurements actually measure.

时间分辨角分辨光电子能谱是对远离平衡的材料进行最强大的泵浦探针测量之一。与频率相关响应函数与时间无关的线性响应机制不同，在远离平衡的实验中，响应函数在时域中取决于两次。在这项工作中，我们描述了如何使用与时间相关的频率响应函数以及它们如何涉及来自彼此接近的时间的贡献。这意味着不应将它们视为单一确定时间的频率相关响应。相反，傅立叶不确定性关系表明它们涉及来自时间范围的贡献，必须从这个角度来解释。