Wed, 14.07.2021  |  14:00
Virtual, via Zoom link

Visualizing ultrafast dynamics in nanoparticles with Coherent Diffraction Imaging

Prof. Dr. Daniela Rupp | ETH Zürich

talks about

High-intensity short-wavelength pulses from free-electron lasers (FELs) and high harmonic sources (HHG) have enabled novel experiments with extreme spatio-temporal resolution. One key example is coherent diffraction imaging (CDI), where an isolated nanometer-scale specimen is irradiated by an intense extreme ultraviolet (XUV) or X-ray pulse and the elastically scattered photons form an interference pattern which can be measured with a wide-area detector. From this pattern, the structural information can be regained using iterative phase retrieval or forward fitting methods. This enables the in-situ study of fragile specimen as single viruses, aerosols, and even superfluid helium nanodroplets which cannot be deposited and analyzed e.g. with other microscopy methods.

In time-resolved configurations using the intense short-wavelength pulses as a probe of the dynamics, fundamental questions of light-matter interaction can be tackled in a near-background-free manner on very short length and time scales. If one measures in the vicinity of electronic resonances, even changes in the electronic structure, e.g. due to ionization, can alter the scattering response quite dramatically and can thus in principle be followed with diffraction imaging. However, because electron dynamics occur on a timescale shorter than the typical tens of femtosecond pulse durations of FELs, we expect an exciting development in this context from the current progress at X-ray FELs and HHG sources towards high-intensity attosecond pulses.

In my talk I will discuss recent results using extreme ultraviolet (XUV) pulses for diffraction imaging. The comparably long wavelengths allow for the measurement of wide-angle diffraction patterns that contain three-dimensional information even in a single-shot pattern, thus enabling the 3D structural characterization of superfluid spinning helium droplets and silver nanopolyhedra. Using optical pump pulses with moderate intensities to excite the isolated nanoparticles, we find indications of ultrafast melting and instable phase explosions in superheated silver clusters. On a few-femtosecond and even sub-femtosecond timescale, we observe interesting switching dynamics in the diffraction response of helium nanodroplets. These recent results demonstrate the capabilities of diffraction imaging to visualize ultrafast nanoscale dynamics in highly excited matter.

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