## 4 Outlook
### Angular Streaking--Full Time-Energy SASE Pulse Retrieval
(reproduced with permission from [124], Copyright IOP Publishing, 2017). The vector potential of the streaking field rotates in the lab frame with a well-specified period, like the hand of a clock, and imprints the XFEL pulse temporal structure onto the angularly resolved distribution of streaked photoelectrons [63; 125; 126]. Varying the wavelength of the circular streaking field allows to set the period of the 'clock' and thus the temporal resolution of the measurement. While capable of measuring the shortest X-ray FEL pulses with a resolution of \(\sim\)160 attoseconds at 800 nm, the resolution can easily be adapted to longer pulse durations via the adjustable streaking wavelength. To avoid ambiguities, the period of the streaking laser field has to be larger than the XFEL pulse duration. At the same time, the relative strength of the streaking effect gives information about the arrival time shift from shot to shot. Moreover, this setup allows to investigate the shift of the photoelectron energy versus delay within a single X-ray pulse and thus measure its instantaneous frequency. For the first time, this would provide the direct measurement of residual X-ray pulse chirp from the different electron bunch compression settings, pointing the way toward full XFEL spectral phase control and chirped pulse amplification. The proposed extended and angle-resolving method of streaking can be realized with an array of 16 independently working time-of-flight spectrometers which are aligned in the dipole plane of the incoming light (see Figure 27, left). As described previously, the circularly polarized streaking laser and the XFEL have to be overlapped spatially and temporally in the experimental chamber, where the X-ray pulse ionizes a diluted noble gas e.g., xenon. In the presence of the optical field the original angular distribution of photoelectrons is redistributed according to the rotational angle of the vector potential at the moment of their generation. Thus, photoelectrons generated from different parts within the XFEL pulse envelope are streaked at different angles, effectively mapping time to angle (Figure 27, right). We therefore do not require precise knowledge of the X-ray arrival time on the scale of the carrier phase of the streaking laser, as was the case in past streaking experiments, where temporal overlap with the linear ramp of the linearly polarized vector potential enabled the photoelectron energy shift-to-time mapping. Since the information of the XFEL pulse structure is given through the relative change in angular distribution within each single XFEL shot, we expect this technique to be more robust with regards to pulse energy instabilities and less demanding with regards to the optical pulse shape. Figure 27: Conceptual design of angular streaking. (**a**) Sketch of the experimental chamber with 16 independent time-of-flight spectrometers (reprinted under the terms of the Creative Commons Attribution 3.0 License from [122]). (**b**) The vector potential of the optical streaking laser rotates similar to the hand of a watch. Photoelectrons resulting from different parts of the X-ray pulse are therefore streaked at different angles, allowing the reconstruction of the X-ray temporal profile (reproduced with permission from [127], Copyright Nature Publishing Group, 2011). First demonstration measurements with this setup have successfully been conducted at the Linac Coherent Light Source in California, USA, and the results are being analyzed at the moment by some of the authors of this article. The first impression of the data is very encouraging, giving rise to the expectation of an adjustable online characterization method for complete shot-to-shot arrival time and time-energy X-ray FEL pulse reconstruction in the near future.