## 1 Introduction--Laser-Assisted Time-Domain Characterization at Free-Electron Lasers  
### Availability of ultrashort, high-brightness X-ray pulses  
Exploring fundamental quantum building blocks of nature and their interactions on time scales of molecular, atomic and even electronic motion, has led to the development of scientific instruments with steadily enhanced spatial and temporal resolution. For modern day optical tools, that historical path reaches from the discovery of X-rays and their properties just before the turn to the 20 th century [1], via synchrotron radiation detected as parasitic electron energy-loss effect in the middle of the last century [2], to the parasitic operation at electron synchrotrons [3]. Next came dedicated beamlines built as multiple synchrotron radiation stations around storage rings [4], and finally high-brightness X-ray beams through periodically alternating magnetic insertion devices [5] were achieved. Today, the most recent additions to the arsenal of X-ray physics stem from high-order harmonic generation (HHG) by intense few-cycle optical laser pulses [6] and from the realization of the concept of free-electron lasers (FELs) [7; 8] at large-scale facilities around the world. These FEL sources provide ultrashort high-brightness pulses from the soft X-ray regime, like the free-electron laser in Hamburg (FLASH), operated by DESY in Germany [9] and the free-electron laser radiation for multidisciplinary investigations (FERMI) seeded FEL in Trieste, Italy [10], to the hard X-ray range, e.g., the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in California, USA [11] and the SPring-8 angstrom compact free-electron laser (SACLA) in Japan [12], among others [13; 14]. Future machines, using novel laser-based accelerator concepts promising much more compact X-ray sources [15; 16; 17; 18] or utilizing superconducting linear accelerators for higher repetition rates in the MHz range (e.g., the European XFEL in Germany [19] or LCLS-II in the USA [20]), are expected to generate X-ray pulses with few- to sub-femtosecond durations as well, and are likewise crucially dependent on a precise X-ray pulse metrology.  
Well-controlled X-ray pulses with durations shorter than few tens of femtoseconds that are currently produced at X-ray free-electron lasers (XFELs) are a necessary prerequisite for the investigation of ultrafast processes on the time scale of the motion of bound electrons, as have been successfully studied in seminal high harmonic generation-based attosecond experiments [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. A number of schemes have been proposed and implemented over the last decade to shorten the pulse durations at XFELs to the few-femtosecond or even sub-femtosecond range, many of them based on the principle that the X-ray pulse duration is related to the driver electron bunch length. One approach, the so-called'slotted spoiler', was invented at the LCLS and provides few-femtosecond to sub-femtosecond pulses [32; 33]. The working principle of this scheme is illustrated in Figure 1 on the left. A thin foil with a narrow vertical slit is introduced into the electron beam path in one of the magnetic bunch compressor chicanes and spoils the beam quality (emittance) of the largest part of the spatially chirped electron bunch via Coulomb scattering. Only the small central slice of the bunch passes unhindered through the slit and will subsequently experience efficient FEL amplification in the undulator. Other proposed methods involve the modulation of the electron bunch with a high-power, few-cycle optical laser in a single-period wiggler before the FEL undulator to induce a pronounced energy modulation along the electron bunch and a sub-femtosecond spike in the electron peak current. Thus, only a small region of the bunch contributes efficiently to the X-ray amplification process and generates an attosecond spike of X-ray radiation, see for example [34; 35; 36; 37].