## 3 Linear Streaking Measurements at Free-Electron Lasers
### Experimental implementation  
Proof-of-principle experiments on THz streaking were first performed at the free-electron laser FLASH in Hamburg. This facility operates in the XUV spectral range emitting pulses with \(\sim\)100 femtosecond duration and below. Typical pulse energies are on the order of 100 \(\upmu\)J. The experimental setup is shown in Figure 17.  
Figure 16: Time-of-flight (TOF) Energy resolution at FELs. In contrast to application at synchrotrons, the resolution in time-of-flight spectra collected at XFELs is dependent on the single electron response of the detector. Panel (**a**) depicts the single-electron response; panel (**b**) shows multiple electron hits; panel (**c**) shows the effective resolution, if the multiple electron hits are accumulated within a single-shot acquisition or a cumulative acquisition. At XFELs, the acquisition is single-shot and the observed signal is a convolution of the photoelectron bandwidth with the single-electron response. If the acquisition is cumulative, the observed signal is the convolution of the photoelectron bandwidth and the resolution with which the edge of the single-electron signal can be determined. (Figure from [95]). THz pulses were generated with user pump-probe laser, \(\sim\)3 mJ, 60 fs pulses from a Ti:sapphire laser amplifier by tilted pulse front optical rectification in LiNbO\({}_{3}\). The peak on-target electric field strength was observed to be 165 kV/cm. The THz electric-field shape was characterized independently by electro-optic sampling, and the absolute field strength determined from a distribution of streaking measurements as discussed before. The THz electric field is shown in Figure 18, panel (a). Single-shot streaking measurements on XUV pulses at \(\sim\)260 eV photon energy were made in helium. The corresponding single-shot and averaged THz streaking spectrograms are shown in Figure 18, panels (b) and (c), respectively. To build the spectrogram, the delay between the THz and the soft X-ray pulses was scanned in 100 fs steps. Approximately 400 single-shot spectra were averaged at each time delay to generate the averaged spectrogram. For large delays, where the THz field is weak, the He 1s photoemission peak is nearly unaffected and located near its field-free energy. Close to temporal overlap the kinetic energy of the photoelectrons is shifted and broadened depending on the THz streaking field phase. In the averaged spectrogram the dynamic range given by the single valued THz streaking ramp is indicated with dashed white lines. As the streaking ramp of \(\sim\)600 fs is significantly longer than the maximum expected FEL pulse duration and timing jitter, once the streaking THz pulse and the FEL are temporally overlapped, all single-shot acquisitions will occur on a uniquely defined position on the streaking ramp. Figure 17: THz streaking apparatus. Layout of the THz streaking experimental setup used at FLASH: The NIR laser pulse is split into two parts. Most of the pulse energy is used for the tilted pulse-front THz generation in LiNbO\({}_{3}\), in setup geometry as shown. The remaining 1% pulse energy is used as a probe for electro-optic sampling (EOS) of the THz pulses in a 500 μm thick zinc telluride (ZnTe) crystal. The NIR probe pulse and the FEL pulse pass through a 2 mm hole in the parabolic mirror and are collinearly overlapped with the THz beam and focused into the interaction region, which is fixed by the position of the time-of-flight spectrometer. For the EO-sampling a ZnTe crystal is placed in the exact interaction region. The crystal is also used as an observation screen for monitoring the spatial overlap of the FEL and the NIR probe pulses. For the streaking measurement a gas nozzle providing neon or helium is placed in the interaction region. The FEL pulses eject a burst of photoelectrons from the atomic gas system and the kinetic energy of the photoelectrons is modified by the THz pulse.