The advent of X-ray Free Electron Lasers (FELs) opens the door for unprecedented studies on non-crystallin nanoparticles with high spatial and temporal resolutions. In the recent past, ultrafast X-ray imaging studies with intense, femtosecond short FEL pulses have elucidated hidden processes in individual fragile specimens, which are inaccessible with conventional imaging techniques. Examples include airborne soot particle formation , metastable states in the synthesis of metal nanoparticles  and transient vortices in superfluid quantum systems  . Theoretically, ultrafast coherent diffraction X-ray imaging (CDI) could achieve atomic resolution in combination with sub-femtosecond temporal precision. Currently, the spatial resolution of ultrafast X-ray CDI is limited to several nanometers due to a combination of several factors such as X-ray photon flux, image imperfections and ultimately, sample damage  .
In this talk, I will present several experimental studies, which address these limitations and/or demonstrate the potential of ultrafast CDI. In the first part of the talk, I will report on a novel "in-flight" holographic method which overcomes the phase problem and paves the way for high-resolution X-ray imaging in presence of noise and image imperfections . The second part will focus on potential applications of ultrafast X-ray CDI such as visualization of irreversible light-induced dynamics at the nanoscale with nanometer and sub-femtosecond resolutions . In the third part, I will present world's first diffraction images of heavy atom nanoparticles recorded with isolated soft X-ray attosecond pulses. The study indicates that the combination of the optimal pulse length and X-ray energy can significantly deviate from linear models and control over transient resonances might be an efficient pathway for the improvement of spatial resolution  .
In summary, ultrafast CDI is a powerful method for studies of transient non-equilibrium dynamics at the nanoscale. The increasing number of X-ray FEL facilities, and the constant improvement in accelerator and X-ray focusing technology will broaden our capabilities to observe transient states of matter. This development will have a significant impact on research fields such as catalysis, nanophotonics, matter under extreme conditions, light-matter interactions and biological studies.
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