## 3 Linear Streaking Measurements at Free-Electron Lasers
### Experimental implementation
#### 3.1.2 Laser-Driven Terahertz Streaking for Complete Temporal Characterization  
**Single-cycle terahertz streaking at XFELs**  
In contrast to attosecond HHG sources, at SASE XFELs the temporal profile and arrival time of the X-ray pulse changes from shot to shot and cumulative measurements are typically not possible. Therefore, the pulse measurements should be performed on a single-shot basis. In addition, the duration of SASE FEL pulses can be as long as 100 femtoseconds. As a result, lower frequency streaking fields, in the THz regime, are required (1 THz corresponds to a half-cycle time of 500 fs) as the ionizing FEL pulse must be shorter than a half-cycle of the streaking field. While phase-locked THz pulses do not typically exist at XFEL facilities, at FLASH it is possible to send the \"spent\" FEL-driving electron bunch through an additional dedicated undulator to generate multi-cycle, phase-stable THz pulses. In principle, this allows for direct application of attosecond techniques. Indeed, in previous work, this THz source was used as the streaking field for temporal characterization of the XUV FEL pulses [101]. However, this technique does not provide access to relative timing information for pump-probe experiments. Furthermore, the THz streaking field changes as the electron beam is varied, fluctuates or is tuned, influencing the measurement and effectively limiting the utility of this technique. In an extreme case, when the accelerator is tuned for the shortest FEL pulses using low-charge highly compressed bunches, the beam-based generated THz might not even be strong enough for streaking. An independent laser-driven THz source for pulse characterization overcomes the limitations of streaking with accelerator-based THz sources, while maintaining the capability to characterize 100 fs long pulses. In addition, as laser-driven THz fields are locked to the external laser, the streaking measurement provides the capability to determine simultaneously the FEL pulse arrival time and its pulse profile on a time base that is synchronized to the pump-probe experiment environment. Many laser-based THz sources rely on optical rectification of femtosecond Ti:sapphire laser pulses in a LiNbO\({}_{3}\) crystal [102]. With this technique, high-field phase-stable single-cycle THz pulses with pulse durations of \(\sim\)2 ps can be generated. The THz vector potential, which is the relevant quantity in streaking spectroscopy, has a half-cycle of \(\sim\)600 fs. The half-cycle duration, or streaking ramp gives the dynamic range of the measurement. Notably, it is significantly longer than the maximum expected FEL pulse duration and facility timing jitter. Therefore, once the temporal overlap between the X-ray pulse and the THz pulse has been established, all subsequent FEL pulses will overlap at some point on the single-valued streaking ramp allowing them to be characterized unambiguously. The principle of a single-cycle THz streaking measurement is illustrated in Figure 14. Three characteristic measurement conditions are shown in the figure. In the first panel the FEL and THz pulses are not temporally overlapped, therefore, the photoelectron spectrum remains unperturbed and only the FEL pulse spectrum shifted by the ionizing potential of the target gas is measured. In the second panel the FEL pulse is overlapped with an extremum of the vector potential (it overlaps with the bottom of the streaking ramp), resulting in a maximally downshifted photoelectron spectrum with minimized spectral broadening. Consequently, the temporal structure of the pulse is not observed in the measured spectrum. In the last panel, the ionizing FEL pulse coincides with a zero crossing of the THz vector potential \(A(t)\). Photoelectrons emitted by the leading and trailing edges of the FEL pulse are streaked with a momentum shift of opposite sign, resulting in a maximally broadened photoelectron around the field-free central energy.