X-ray movies of ultrafast melting and boiling in plasmonic nanoparticles

Via single-shot coherent diffractive imaging (CDI), the structure and dynamics of isolated nanosamples can be directly visualized: Intense and short pulses of X-ray free-electron lasers (XFELs) or intense high-harmonic generation (HHG) based sources scatter off a free-flying nanostructure, forming an interference pattern that is captured with a large-area detector. With computer-based iterative phase-retrieval or forward-fitting methods a snapshot of the object’s structure can be retrieved from the pattern. This has opened a door for us to study intense laser-matter interaction with unprecedented detail.

In this seminar, I will give an introduction to the CDI method and discuss its capabilities on the example of the dynamics in laser-heated plasmonic nanoparticles. In a recent experiment at the SwissFEL in Villigen, we  brought silver nanoparticles into the gas phase and heated them with laser pulses tuned to their surface plasmon resonance. This approach allows for a uniform heating of the nanoparticles at comparably low laser intensities, avoiding strong-field effects like tunneling and electron impact ionization. We observe, depending on the heating laser’s intensity, a wide range of processes from surface to full melting, internal boiling, cavitation, expansion and inflation, droplet vibrations, up to explosive boiling. Molecular dynamics simulations show that the systems travel on rather similar trajectories through the phase diagram, differing only in whether and where the stability limit of the metastable superheated liquid is crossed.

These results exemplify the maturity of time-resolved single-particle coherent diffraction imaging for investigating ultrafast dynamics in matter, being now on a level where we can really extract previously inaccessible physical quantities of matter under extreme conditions.