Breaking resolution limits in ultrafast X-ray diffractive imaging

Introduction

Our ability to observe processes and study function at the nanoscale is hindered by the compromise between temporal and spatial resolutions inherent to the majority of far-field imaging techniques. This limits our perspective on a wide range of non-equilibrium processes at the nanoscale such as:

• Chemical/catalytic reactions
• Ultrafast phase transitions
• Biological processes at room temperature in native phase

XFEL Technology

Intense and spatially coherent femtosecond-short X-ray flashes from free-electron laser (XFEL) sources can combine high spatial and temporal resolutions through 'diffraction-before-destruction' coherent diffractive imaging (CDI) of individual nano-specimens within a single exposure.

XFEL CDI studies have found surprising varieties of morphologies in soot, unknown metastable shapes of metal nanoparticles, and exotic states of water, which are otherwise inaccessible. The principal investigator (PI) and colleagues applied this technique to follow an ultrafast irreversible laser-superheating process with few nanometers spatial and 100 femtosecond temporal resolutions at the single nanoparticle level.

Challenges and Proposal

Despite significant efforts, the spatial resolution of single XFEL CDI images of non-periodic specimens could not be improved beyond a few nanometers. This proposal will overcome this limit by exploiting previously little-explored phenomena which arise when specimens are exposed to newly available intense 500 attosecond to few femtosecond short FEL pulses.

All matter exposed to intense X-rays is photo-ionized. When XFEL pulses are comparable to or shorter than subsequent relaxation processes, non-linear effects such as:

1. Transient resonances
2. Resonant stimulated emission

increase the brightness of images by several orders of magnitude and significantly improve the spatial resolution.

In combination with sparsity-based reconstruction algorithms, this proposal will push ultrafast CDI towards the single macromolecule limit and open novel avenues for photochemistry, catalysis, and material studies.