Clocking Femtosecond X Rays

Abstract

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms. DOI: 10.1103/PhysRevLett.94.114801 PACS numbers: 41.60.Cr, 41.75.Ht, 42.65.Re Ultrafast x-ray pulses are providing our first view of subpicosecond atomic motion. New sources based on high harmonic generation [1,2] and laser-produced plasmas [3] as well as femtosecond laser-sliced synchrotron emission [4] have been demonstrated. These sources produce x-ray pulses with durations of less than a few hundred femtoseconds, the time scale of vibrations in solids and molecules and the making and breaking of chemical bonds. While these sources provide the time resolution necessary to study these dynamics, their relatively low brightness limits their application and often hinders attempted experiments.A new generation of linear-accelerator-based x-ray free electron lasers (XFELs) will be more than 20 orders of magnitude brighter than laser-plasma-based sources and have the potential to produce x-ray pulses below one femtosecond in duration [5]. With x rays from an XFEL, researchers can expect to image chemistry in real time on the atomic scale. While these new XFELs will be far brighter than any other ultrafast x-ray source, their physical size and complexity introduce new challenges which, if left unaddressed, will restrict their application. A major obstacle will be the inability to precisely synchronize the time-dependent process being studied with the x-ray pulse generated by a large accelerator-based source.Subpicosecond time-dependent phenomena are typically studied with pump-probe techniques in which the dynamics are initiated by an ultrafast laser or laser-driven source and then probed after a time delay. If these experiments can be self-synchronized, with the pump and probe having a common laser source, then precise time delays can be produced using different optical path lengths. The time resolution is then limited by the overlap of the pump and probe pulses that can be as short as a fraction of a PRL 94, 114801 (2005) P H Y S I C A L

A femtosecond X-ray/optical cross-correlator

“…The time scale can be deduced from a measurement shown in Fig. 2 c), where we corrected for the major sources of temporal jitter within the accelerator (typically 0.25 ps RMS 27 ) using electro optical sampling 34 to determine the arrival time of selected electron bunches relative to the optical laser. When we fit the reflectivity transient with an exponentially decaying response function convoluted with a Gaussian we obtain a full width at half maximum of 160 ± 44 fs.…”

“…An important first step has been achieved by measuring the relative arrival time of the electron bunches within the linear accelerator structure through electro optical sampling (EOS) 34 .…”

“…c) The width of the initial drop in optical reflectivity as determined fitting an exponentially decaying response function convoluted with a Gaussian (full width at half maximum of 160 ± 44 fs) is limited by the pulse length of the optical laser pulse (120 -150 fs). To eliminate temporal jitter from the accelerator the delay is corrected for the electron bunch arrival determined with electro optical sampling (EOS) 34 . We gratefully acknowledge support by the scientific and technical staff of the FLASH facility, in particular Stefan Düsterer, Harald Redlin and Rolf Treusch.…”

Time-resolved imaging using x-ray free electron lasers

“…By combining this technique with the shortest available FEL wavelengths [1] and proven synchronization methods [14] this technique could be straightforwardly extended to sub-nm spatial and sub-100 fs temporal resolution.…”

Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers

“…High-resolution pump-probe experiments require a precisely characterized FEL pulse and precise synchronization to an external optical pump-probe laser. While so far different schemes for arrival time measurements have been developed [11][12][13][14][15][16][17][18][19][20][21][22][23] , insufficient characterization of the XUV or X-ray FEL pulse duration at the experimental station remains the main handicap of single pass free-electron lasers that rely on self-amplified spontaneous emission (SASE).…”