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Ultrafast Dynamics using ATTosecond and XUV Free Electron Laser Sources

Final Report Summary - ATTOFEL (Ultrafast Dynamics using ATTosecond and XUV Free Electron Laser Sources)

Two recently developed ultrafast XUV/x-ray sources offer fundamentally new tools for the investigation of atomic-scale structure and dynamics: High-harmonic attosecond XUV pulses allow tracking of ultrafast electron motion in real time, while XUV/x-ray Free Electron Lasers (FELs) allow resolving structural changes in (bio-) molecules through femtosecond time-resolved x-ray diffraction. The objectives of the ATTOFEL Network, established in 2009, were to: (1) establish collaboration in the field of attosecond science, (2) foster collaboration between the attosecond and FEL communities, (3) train a generation of young scientists, (4) increase the competitive advantage of European attosecond and FEL facilities, (5) strengthen European companies, and (6) strengthen the international research community in this field. The ATTOFEL network ceased activities at the end of November 2013.
The scientific goals (objectives 1, 2, 4, 5, and 6) of ATTOFEL were addressed in three work packages and included 15 scientific milestones. At the end of the 2nd period all milestones had been achieved (see table in this report) and in many cases the scientific output greatly exceeded the envisaged milestones. A large number of scientific results were published in first-tier journals, indicating the high relevance of the scientific activities at all partner sites.
To achieve the training goals (objectives 3 and 6) the network created a comprehensive and structured learning environment of local and network-wide training opportunities. A printed network toolkit that was published in the first reporting period, the definition of individualized career development plans, and the establishment of a structured website raised the training awareness of the fellows and partners. A summer school in May 2011 offered intensive training on all aspects of ultrafast, strong-field, and XUV physics. The lectures presented at this school formed the basis for a book that was recently published by Wiley (“Attosecond and XUV Spectroscopy: Ultrafast Dynamics and Spectroscopy”, edited by Thomas Schultz and Marc Vrakking, ISBN: 978-3-527-41124-5, published in January 2014). A winter school that was organized by the fellows in February 2013 complemented this training with a focus on career choices, and featured eminent speakers with academic and corporate backgrounds. Throughout, non-scientific “soft-skill” training modules complemented the scientific training program.
Efficient knowledge exchange has consistently been a prerequisite for the accomplishment of all objectives and was fostered in a total of 8 network meetings that were organized at or near partner locations. The final network meeting was combined with the 4th International Attosecond Physics Conference (“ATTO IV”), where the first day of the conference was devoted to the network research as presented by the young researchers. This greatly helped maximizing the impact of the ATTOFEL activities and created new opportunities for networking in the larger scientific community. The high level of scientific exchange is reflected in a large number of collaborative research projects and corresponding publications.
The body of scientific work performed within the Network is too large to be addressed in this format. We therefore only present a number of highlights, which reflect the rapid scientific progress and the high degree of collaboration within the network.

- Researchers from AMOLF and MBI probed the dynamics of electron-ion recollision with intense 7-micrometer laser pulses from a free-electron laser [Science 331, 61 (2011)].
- Researchers at CEA and CNR used two distinct techniques to observe ultrafast electron dynamics in nitrogen: Tomographic imaging provided an attosecond time-resolved image of the electronic hole left by coherent ionization [Nature Physics 6, 200 (2010)] and an XUV-IR pump-probe experiment controlled a dissociative ionization process.
- XUV pump–probe experiments of FORTH and MPQ probed rapidly evolving atomic coherences on the 1 fs scale [Nature Physics 7, 781 (2011)]. The experiment showcases a novel tool for the direct observation of electronic motion on femtosecond and attosecond timescales.
- A joint FLASH-FEL beam-time of the Lund and MBI partners revealed structural dynamics in aligned molecules with the help of a velocity map imaging spectrometer.
- At CEA, single-shot and multi-shot coherent diffraction imaging was performed on three-dimensional nanometer scale objects and on a magnetic scattering ring.
- Partners at CNR used ionization gating and shaping of few-optical-cycle pulses to increase the attainable intensity of isolated attosecond pulses by several orders of magnitude, into the nanojoule range [Nature Photonics 4, 875 (2010); Nature Photonics 5, 655 (2011)].
- Partners at MPQ demonstrated the light field synthesis of intense non-repeating waveforms confined to sub-cycle (<2.7 fs) time intervals. The sculpted pulses open new prospects for steering the atomic-scale motion of electrons with the electric force of light.
- FEMTOLASERS developed a novel high-power oscillator and implemented a feed-forward CEP scheme based on pioneering work at the MBI. The concept was tested at the MPQ node and delivered a new precision record for the CEP stabilisation of a Titanium-Sapphire laser. The transfer of this technology to power-scalable laser sources may lead to attosecond sources at MHz repetition rates.
- Theoretical work at the Szeged node proposed more efficient mechanisms for HHG generation. The mechanisms will be experimentally verified at collaborating nodes in Lund and at the MBI.
- Researchers from FORTH successfuly implemented the first XUV-pump-XUV-probe study of 1 fs scale molecular dynamics.
- An optical experimental setup for measuring the pulse duration and the temporal substructure of pulses from an XUV FEL was designed and tested at the UHAM node. Additionally, the focal region at FLASH could be imaged, using a species-selective mode of a specially constructed ion microscope.
- In collaboration with researchers from the Queens University Belfast, Researchers at the CNR node performed the first measurement of ultrafast charge migration in a biomolecular building block (phenylalanine) by using attosecond pulses. Relatedly, the MBI group successfully performed experiments on a series of polycyclic aromatic hydrocarbons (PAHs).
- The rapid development of attosecond technologies in the network culminated in increasingly complex attosecond experiments involving larger molecules at the CNR, MBI, and Lund nodes, including the observation of ultrafast dynamics in nitrogen, CO2, and C2H4 molecules.
- Several methods for an order-of-magnitude enhancement of the HHG process were predicted by the Szeged node and became subject of collaborative experiments at partner sites (MBI, PSI) and external sites (Cluj-Napoca, Budapest).
- The CEA group performed an advanced characterization of the attosecond emission from aligned molecules by combining spectrally-resolved and spatially-resolved interferometric measurements in exactly the same generation conditions.
- MBI and Amplitude Technologies built the first Terawatt-class CEP-stable Ti:Sapphire amplifier.
- Using the newly-developed high power CEP-stabilized laser oscillators developed in the first period, Femtolasers achieved tremendous progress on the precision and long-term reliability of CEP-stability in amplified laser pulses. This work happened in collaboration with MPQ (for high average power sources), CNR-IFN (for hollow-fiber compression) and the French company Fastlite.

The ATTOFEL Toolkit (a 78-page introductory book on the network activities) and an interactive Website fostered communication and interaction among the fellows and partners. The network organized eight Network meetings and both a summer and winter school with intensive programs of scientific seminars, soft-skill seminars, and hands-on training. The resulting expertise of all fellows was reflected in the high quality of their experimental and theoretical work. As mentioned, the summer school led to the publication of a graduate level textbook.

Extraordinary science was performed in the course of the ATTOFEL Network activities. The scientific output greatly exceeded the planned deliverables. This outcome was the product of strong collaborative ties, rapid knowledge exchange and effective training efforts. The exchange of technological expertise between the attosecond and FEL communities created important benefits for all participating partners. The industrial partners were quick to commercialize novel technologies for the mutual benefit of all partners, and beyond. The competitive position of attosecond and FEL research in Europe was clearly strengthened.
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