Abstract
For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time–energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.
Highlights
Laser-Assisted Time-Domain Characterization at Free-Electron LasersAvailability of ultrashort, high-brightness X-ray pulsesExploring 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 20th 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]
Studies of two-color photoionization of atoms in weak resonant optical/NIR fields are well established at synchrotron radiation facilities [70], it is only with the availability of short wavelength free-electron lasers (FELs) that experiments have been performed, in which electrons ejected by the XUV or
Summary
Two-color experiments using the combination of intense free-electron laser femtosecondphotoionization pulses in the extreme ultravioletusing (XUV)the to X-ray wavelength andfree-electron optical or NIR laser combination ofrange intense pulses offer great potential to determine the temporal characteristics, i.e., pulse durations and the femtosecond pulses in the extreme ultraviolet (XUV) to X-ray wavelength range and optical or NIR relative temporal arrival time, of the FEL pulses. For the first experiment using linearly polarized FEL and NIR pulses, it was demonstrated that the sideband intensity depends in a sensitive and characteristic way on the relative orientation between the two linear polarization vectors [88] Since this dichroic effect is caused by a different ratio of the partial waves contributing to the outgoing electron (e.g., s- and d-waves in case of a two-photon ionization in He), which can be calculated very precisely by theory, it can in turn be used to determine the degree of polarization of the FEL beam.
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