Bright ultrashort soft X-ray pulses for carbon K-edge spectroscopy

Some of the most informative types of spectroscopy revealing electronic structure and interaction in the condensed phase are based on the use of X-ray beams typically delivered by synchrotron sources. A set of powerful approaches analyzes the fine absorption structure near element-specific absorption edges of C, N, O and metals. Spectral lines, their width and splitting, captured in the steady-state absorption and secondary electron emission spectra, indicate a femtosecond to attosecond scale of the underlying electronic dynamics. Consequently, time- resolving this structure with X-ray pulses of appropriately high temporal resolution constitutes a quest very similar to the one faced by the discipline of femtochemistry over 25 years ago. Over the first decade since their emergence, attosecond soft X-ray sources have undergone an immense transformation in terms of isolated pulse duration that was pushed below 70 attoseconds by double optical gating, and also the photon flux could be substantially increased. However, all these gains were limited to photon energies below 100 eV, outside the most interesting spectroscopic region. The aim of this proposed research is to pave the way for the establishment of ultrafast spectroscopy in the soft X-ray region, which contains many absorption edges of atomic elements such as the carbon K-edge at 285 eV. The coherent soft X-ray light source required for this purpose is based on high harmonic generation (HHG) and cutting-edge laser technologies in the near and mid infrared (IR). Ultrafast soft X-ray spectroscopy requires high photon flux, which can be realized by advanced laser technologies, such as intense mid-IR light sources as pioneered by the proposers group. The HHG source that will be developed in the framework of this three-year project is based on a laser-pulse frequency synthesizer to combine multi-color electric fields. The synthesizer is driven by a home-made Yb amplifier system capable of delivering femtosecond pulses with unprecedented energy content at a kHz repetition rate. Successful completion of the laser and HHG source will enable the production of bright, isolated few- to sub-femtosecond soft X-ray pulses carried at photon energies covering the carbon K-edge delivered at a kHz repetition rate. The existence of soft X-ray pulses with such parameters opens the door to time-resolved carbon K-edge spectroscopy in the few- to sub-femtosecond dynamical regime, as will be demonstrated by proof-of-principle experiments that investigate ultrafast carrier scattering and heating dynamics in graphene. If successful, this project may provide a tabletop-scale answer to a wide community currently relying on synchrotrons. Moreover, it will be able to achieve a time resolution currently projected for multibillion international facilities under construction.

Some of the most informative spectroscopic techniques for investigating the electronic structure and interaction in condensed matter rely on the availability of X-rays typically derived from synchrotrons. Among the very potent investigation methods is the analysis of fine absorption structures in the vicinity of element-specific absorption edges of C, N, O and metal atoms. A time-resolved measurement of the spectral structure with X-rays of a suitably short duration represents an analogous challenge with optical pulses which Femtochemistry faced 30 years earlier. Within a decade of initial demonstration, the attosecond sources in the soft-X-ray frequency range have reached spectacular improvement both with respect to the achievable pulse duration, which was pushed under 70 fs, and the photon flux. However, all these improvements have been restricted to the photon energy range up to 100 eV-the spectral range practically outside the main interest for spectroscopy. The goal of this project was to establish ultrafast soft-X-ray spectroscopy. At the project submission stage, we intended to scale up the X-ray spectrum obtainable via higher order harmonic generation (HHG) up to the carbon K-edge at 286 eV in order to enable transient absorption spectroscopy on molecules containing carbon. However, other research groups have reached the C K-edge with an adequately high photon flux already at the start of our project and demonstrated first time-resolved absorption measurements by 2016. Correspondingly, our research goals were modified to achieve, alongside the planned molecular agenda, a sufficient photon flux to demonstrate time-resolved magnetic scattering in a solid target. Because of a very low scattering cross-section, this method demands a short-pulse X-ray source that is by orders of magnitude brighter than the one sufficient for absorption measurements. The HHG source developed in this project is based on a nonlinear-optical post-compression scheme that broadens the spectrum of the input 1030-nm laser pulses, red-shifts the spectrum if required, and compresses the pulse duration below 20 fs. Our tabletop X-ray system, consisting of a laser amplifier and an HHG source, delivers extremely bright femtosecond X-ray pulses. As a project highlight, we have pushed for high peak brightness and photon flux in a 5-eV-wide spectral window at 155 eV, at the N-edge of Tb. With this newly developed coherent X-ray source we succeeded in recording the world-first diffraction movie of the magnetic domain recovery in a Tb-doped ferromagnetic in a compact setup constructed on a conventional-size laser-table. The development of this new functionality for a laboratory-type laser system will create competition to facility-type X-ray sources such as synchrotrons. Moreover, our lab-scale setup, owing to the combination of optical excitation and X-ray probing with short pulses, offers already in the present form a femtosecond time resolution, which remains currently out of reach for conventional synchrotron sources.