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@PHDTHESIS{Zhang:168069,
      author       = {Zhang, Dongfang},
      title        = {{F}emtosecond {S}tructural {D}ynamics on the {A}tomic
                      {L}ength {S}cale},
      school       = {University of Hamburg},
      type         = {Dissertation},
      reportid     = {DESY-2014-02322, DESY-THESIS-2014-005},
      pages        = {126},
      year         = {2014},
      note         = {Dissertation, University of Hamburg, 2013},
      abstract     = {This thesis reports on the development and application of
                      two different but complementary ultrafast electron
                      diffraction setups built at the Max Planck Research
                      Department for Structural Dynamics. One is an ultra-compact
                      femtosecond electron diffraction (FED) setup (Egun300),
                      which is currently operational (with a maximum electron
                      energy of 150 keV) and provides ultrashort (~300 fs) and
                      bright (~10 $e/um^(2))$ electron bunches. The other one,
                      named as Relativistic Electron Gun for Atomic Exploration
                      (REGAE) is a radio frequency driven 2 to 5 MeV FED setup
                      built in collaboration with different groups from DESY.
                      REGAE was developed as a facility that will provide high
                      quality diffraction with sufficient coherence to even
                      address structural protein dynamics and with electron pulses
                      as short as 20 fs (FWHM). As one of the first students in
                      Prof. R.J. Dwayne Miller’s group, I led the femtosecond
                      (fs) laser sub-group at REGAE being responsible for the
                      construction of different key optical elements required to
                      drive both of aforementioned FED systems. A third harmonic
                      generation (THG) and a nonlinear optical parametric
                      amplifier (NOPA) have been used for the photo-generation of
                      ultrashort electron bursts as well as sample laser
                      excitation. Different diagnostic tools have been constructed
                      to monitor the performance of the fs optical system. A fast
                      autocorrelator was developed to provide on the fly pulse
                      duration correction. A transient-grating frequency-resolved
                      optical gating (TG-FROG) was built to obtain detail
                      information about the characteristics of fs optical pulse,
                      i.e. phase and amplitude of its spectral components. In
                      addition to these optical setups, I developed a fs optical
                      pump-probe system, which supports broadband probe pulses.
                      This setup was successfully applied to investigate the
                      semiconductor-to-metal photoinduced phase transition in
                      vanadium dioxide and the ultrafast photo-reduction mechanism
                      of graphene oxide.In regard to FED setups, I have been
                      deeply involved in their development. I performed the first
                      study in our compact FED system. I studied the optical and
                      structural response of alkali halides to intense UV
                      excitation conditions, i.e. above the damage threshold of
                      the samples which required the application of a single-shot
                      scheme. In order to gain a better understanding of the
                      ablation process that follows fs optical excitation in
                      alkali halides, I applied a variety of different techniques.
                      Optical reflectivity, femtosecond electron diffraction, ion
                      detection and crater measurements revealed the existence of
                      a cold ablation process that occurs well below the threshold
                      for plasma formation and even that for the melting point of
                      the salts. This atypical cold explosion owes to the presence
                      of highly localized excitonic states and reflects the
                      repulsive nature of initial electronic correlations at
                      play.In the case of REGAE, we performed the first
                      time-resolved experiment following the fs laser heating
                      dynamics and partial melting of polycrystalline gold films.
                      This experiment was crucial to test the overall
                      synchronization of our REGAE machine. We were able to
                      observe a clear dynamics under single-shot photo-excitation
                      conditions and found time zero within 1 picosecond. Further
                      electron pulse characterization will involve the
                      implementation of ponderomotive scattering. I have already
                      constructed the required modular setup and performed all
                      preliminary ASTRA N-body simulations.},
      keywords     = {Dissertation (GND)},
      cin          = {MPSD},
      cid          = {I:(DE-H253)MPSD-20120731},
      pnm          = {Experiments at CFEL (POF2-544) / REGAE - Relativistic
                      Electron Gun for Atomic Exploration (POF2-515)},
      pid          = {G:(DE-H253)POF2-CFEL-Exp.-20130405 /
                      G:(DE-H253)POF2-REGAE-20130405},
      experiment   = {EXP:(DE-H253)CFEL-Exp-20150101 /
                      EXP:(DE-H253)REGAE-20150101},
      typ          = {PUB:(DE-HGF)29 / PUB:(DE-HGF)11},
      url          = {https://bib-pubdb1.desy.de/record/168069},
}