Ultrafast electronic charge redistribution processes in ionic crystals by femtosecond x-ray diffraction
Final Report Abstract
In the present project we aimed at an experimental study of ultrafast redistribution processes of electronic charge in ionic crystals. All experiments were performed using homebuilt tabletop femtosecond x-ray sources. In the femtosecond x-ray diffraction experiments we used 3 different experimental techniques: (i) femtosecond x-ray powder diffraction (Debye-Scherrer method), (ii) femtosecond x-ray diffraction on single Bragg reflections, and (iii) femtosecond x-ray diffraction using the rotation method. On the way to the ultimate goal of the project, i.e. the investigation of the ultrafast dynamics of soft modes in ferroelectrics we mastered important milestones consisting of: Theoretical activities: Explaining the quasi-instantaneous electronic response of low Z materials experimentally found in the first funding period. In particular the unexpected field-induced charge transfer from cation to anion in LiH originates from strong electron correlations as is evident from a comparison with quasi-particle bandstructures calculated within the Coulomb-hole-plus-screened exchange (COHSEX) formalism. Further we presented a detailed theoretical analysis of hard x-ray generation by nonperturbative interaction of ultrashort light pulses with a metal. The Kα flux is theoretically predicted to be proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation. Technological improvements of the femtosecond x-ray diffractometer: To address the strong quest for a higher X-ray flux, we developed in collaboration the group of A. Baltuška in Vienna the first hard X-ray plasma source driven by intense mid-infrared sub-100-fs pulses at 3.9 μm. We discussed all relevant aspects which allow for shot-noise-limited diffraction experiments with table-top femtosecond hard x-ray sources. An essential result of our noise analysis is that a combination of femtosecond high-repetition laser drivers providing a high average optical flux with a detection scheme at the few-photon level provides the highest detection sensitivity of pump-probe experiments. We aim at implementing this strategy with the help of femtosecond mid-infrared driver systems within the year 2018. Femtosecond x-ray diffraction experiments on ferroelectrics: We investigated the generation of acoustic strain waves generated by optical multi-photon excitation of a ferroelectric. The response of the crystal lattice to an electric shift current induced via the two-photon bulk-photovoltaic effect is studied in a lithium niobate (LiNbO3) crystal using femtosecond x-ray diffraction. In particular we demonstrated a four-fold symmetry of the sound velocity of generated acoustic waves as a function of the polarization of the exciting laser and the polar c-axis of LiNbO3. In our most recent femtosecond x-ray diffraction experiments we investigated the relation between the transient electron density ρ(r,t) and the resulting macroscopic polarization P(t). In the prototypical ferroelectric ammonium sulfate [(NH4)2SO4], we determined transient charge density maps using femtosecond x-ray powder diffraction after ultrafast excitation of coherent phonons. In the ferroelectric phase we discovered for the first time a new soft-mode with a 3 ps period. The concomitant macroscopic polarization shows a full reversal of polarity. Such studies have been essentially performed and analyzed in the year 2017.
Publications
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Femtosecond x-ray diffraction maps field-driven charge dynamics in ionic crystals, Faraday Discussions 171 (2014) 373-392
M. Woerner, M. Holtz, V. Juvé, T. Elsaesser, A. Borgschulte
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High-brightness table-top hard X-ray source driven by sub-100- femtosecond mid-infrared pulses, Nat. Phot. 8 (2014) 927-930
J. Weisshaupt, V. Juvé, M. Holtz, S.A. Ku, M. Woerner, T. Elsaesser, S. Ališauskas, A. Pugžlys, A. Baltuška
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Perspective: Structural dynamics in condensed matter mapped by femtosecond x-ray diffraction, J. Chem. Phys. 140 (2014) 020901/1-11
T. Elsaesser, M. Woerner
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Theoretical analysis of hard x-ray generation by nonperturbative interaction of ultrashort light pulses with a metal, Structural Dynamics 2 (2015) 024102/1-19
J. Weisshaupt, V. Juvé, M. Holtz, M. Woerner, T. Elsaesser
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Shift-currentinduced strain waves in LiNbO3 mapped by femtosecond x-ray diffraction, Phys. Rev. B 94 (2016) 104302/1-7
M. Holtz, C. Hauf, A. Hernández, R. Costard, M. Woerner, T. Elsaesser
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Towards shotnoise-limited diffraction experiments with table-top femtosecond hard x-ray sources, Structural Dynamics 4 (2017) 054304/1-11
M. Holtz, C. Hauf, J. Weisshaupt, A. Hernandez, M. Woerner, T. Elsaesser
