Design and test of a broadband split-and-delay unit for attosecond XUV-XUV pump-probe experiments

Attosecond stable dispersion-free delay line for easy ultrafast metrology

We demonstrate a dispersion-free wavefront splitting attosecond resolved interferometric delay line for easy ultrafast metrology of broadband femtosecond pulses. Using a pair of knife-edge prisms, we symmetrically split and later recombine the two wavefronts with a few tens of attosecond resolution and stability and employ a single-pixel analysis of interference fringes with good contrast using a phone camera without any iris or nonlinear detector. Our all-reflective delay line is theoretically analyzed and experimentally validated by measuring 1st and 2nd order autocorrelations and the SHG-FROG trace of a NIR femtosecond pulse. Our setup is compact, offers attosecond stability with flexibility for independent beam-shaping of the two arms. Furthermore, we suggest that our compact and in-line setup can be employed for attosecond resolved pump-probe experiments of matter with few-cycle pulses.

XUV-beamline for attosecond transient absorption measurements featuring a broadband common beam-path time-delay unit and in situ reference spectrometer for high stability and sensitivity

Measuring bound-state quantum dynamics, excited and driven by strong fields, is achievable by time-resolved absorption spectroscopy. Here, a vacuum beamline for spectroscopy in the attosecond temporal and extreme ultraviolet (XUV) spectral range is presented, which is a tool for observing and controlling nonequilibrium electron dynamics. In particular, we introduce a technique to record an XUV absorption signal and the corresponding reference simultaneously, which greatly improves the signal quality. The apparatus is based on a common beam path design for XUV and near-infrared (NIR) laser light in a vacuum. This ensures minimal spatiotemporal fluctuations between the strong NIR laser and the XUV excitation and reference beams, while the grazing incidence optics enable broadband spectral coverage. The apparatus combines high spectral and temporal resolution together with an increase in sensitivity to weak absorption signatures by an order of magnitude. This opens up new possibilities for studying strong-field-driven electron dynamics in bound systems on their natural attosecond time scale.

XUV double-pulses with femtosecond to 650 ps separation from a multilayer-mirror-based split-and-delay unit at FLASH

Extreme ultraviolet (XUV) and X-ray free-electron lasers enable new scientific opportunities. Their ultra-intense coherent femtosecond pulses give unprecedented access to the structure of undepositable nanoscale objects and to transient states of highly excited matter. In order to probe the ultrafast complex light-induced dynamics on the relevant time scales, the multi-purpose end-station CAMP at the free-electron laser FLASH has been complemented by the novel multilayer-mirror-based split-and-delay unit DESC (DElay Stage for CAMP) for time-resolved experiments. XUV double-pulses with delays adjustable from zero femtoseconds up to 650 picoseconds are generated by reflecting under near-normal incidence, exceeding the time range accessible with existing XUV split-and-delay units. Procedures to establish temporal and spatial overlap of the two pulses in CAMP are presented, with emphasis on the optimization of the spatial overlap at long time-delays via time-dependent features, for example in ion spectra of atomic clusters.

Generation of Gigawatt Circularly Polarized Attosecond-Pulse Pairs

A novel scheme for generating a pair of gigawatt attosecond pulses by coherent Thomson scattering from relativistic electron sheets is proposed. With a circularly polarized relativistic laser pulse, the scattered x-ray signal can have a saddlelike temporal profile, where the lower electromagnetic frequencies are found mostly in the center region of this saddlelike profile. By filtering out the latter, we can obtain two few-attosecond pulses separated by a subfemtosecond interval, which is tunable by controlling the energy of the sheet electrons. Such a pulse pair can be useful for an attosecond pump probe at an unprecedented time resolution and for ultrafast chiral studies in molecules and materials.