Ultrashort Free-Electron Laser X-ray Pulses

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

Cascaded regime of optical parametric amplification for efficient terahertz generation

The efficient generation of multi-cycle coherent THz radiation is ofprimary importance to drive novel table-top FELs. We explore thecascaded OPA, a new regime of OPA. We start from a home-builtYb:YLF amplifier at 1017-1020 nm temporally chirped to 800-ps. Wegenerate intra-band DFG in a PPLN crystal phase matched for 0.5THz. The interaction of the IR pulses with the THz pulses producesseveral spectrally cascaded orders, allowing for the generation of THzradiation beyond the Manley-Rowe limit. This technique promises IRto-THz conversion efficiencies in the order of 1-10%. After studying thesystem at room temperature, we explored the case of a crystal at 77 K,which provides much lower THz absorption. The experimental resultssupport the theoretical findings, and reveal that improvements in laserbandwidth and dispersion will lead to the calculated efficiencies

Synthesized Light Transients

Manipulation of electron dynamics calls for electromagnetic forces that can be confined to and controlled over sub-femtosecond time intervals. Tailored transients of light fields can provide these forces. We report on the generation of subcycle field transients spanning the infrared, visible, and ultraviolet frequency regimes with a 1.5-octave three-channel optical field synthesizer and their attosecond sampling. To demonstrate applicability, we field-ionized krypton atoms within a single wave crest and launched a valence-shell electron wavepacket with a well-defined initial phase. Half-cycle field excitation and attosecond probing revealed fine details of atomic-scale electron motion, such as the instantaneous rate of tunneling, the initial charge distribution of a valence-shell wavepacket, the attosecond dynamic shift (instantaneous ac Stark shift) of its energy levels, and its few-femtosecond coherent oscillations