Ultrafast single-shot diffraction imaging of nanoscale dynamics

Abstract

The transient nanoscale dynamics of materials on femtosecond to picosecond timescales is of great interest in the study of condensed phase dynamics such as crack formation, phase separation and nucleation, and rapid fluctuations in the liquid state or in biologically relevant environments. The ability to take images in a single shot is the key to studying non-repetitive behaviour mechanisms, a capability that is of great importance in many of these problems. Using coherent diffraction imaging with femtosecond X-ray free-electron-laser pulses we capture time-series snapshots of a solid as it evolves on the ultrafast timescale. Artificial structures imprinted on a Si 3 N 4 window are excited with an optical laser and undergo laser ablation, which is imaged with a spatial resolution of 50 nm and a temporal resolution of 10 ps. By using the shortest available free-electronlaser wavelengths 1 and proven synchronization methods 2 this technique could be extended to spatial resolutions of a few nanometres and temporal resolutions of a few tens of femtoseconds. This experiment opens the door to a new regime of time-resolved experiments in mesoscopic dynamics.To date, optical pulses have made it possible to resolve dynamics on the femtosecond timescale, but the spatial resolution of these studies has been limited to a few micrometres 3 . On the other hand, femtosecond X-ray pulses have been used to detect Å ngstrom-scale atomic motions in extended crystalline materials with long-range order through time-resolved diffraction experiments 4,5 . An entirely different methodology is needed to investigate the ultrafast dynamics of non-crystalline materials at nanometre length scales. Applications at these scales can be found in the study of fracture dynamics, shock formation, spallation, ablation, and plasma formation under extreme conditions. In the solid state it is desirable to directly image dynamic processes such as nucleation and phase growth, phase fluctuations and various forms of electronic or magnetic segregation.Electron microscopes can provide nanometre to atomic resolution, and have recently been demonstrated with ultrafast pulses 6 . However, they have limited penetrating power and struggle to obtain high-quality single-shot images due to space-charge issues 7 . Synchrotron beams from third-generation sources have comparatively long pulse lengths of 10-100 ps, as determined by the shortest electron bunch length possible in a given storage ring. Synchrotron sources can produce short-pulse ($100 fs) X-rays when operated as femtosecond slicing sources 8 , but produce comparatively weak X-ray beams of 1 Â 10 7 photons per second. Along with X-ray pulses from femtosecond laser plasma sources 9,10 and high-harmonic-generation sources 11 this limits their use to non-destructive phenomena where weak signals can be accumulated over many repeatable excitations of the sample.The intense femtosecond X-ray pulses from free-electron-laser (FEL) sources provide the penetrating power, spatial resolution and single-shot imaging c...

Time-resolved imaging using x-ray free electron lasers

“…. This propagation of disorder corresponds to speed of the explosion, and is consistent with the speed of sound in the heated membrane, calculated to be between 4,000 and 6,000 ms −1 [7].…”

“…Image reconstruction was carried out by phase retrieval using our iterative transform algorithm, Shrinkwrap 8 (see the Methods section). Unlike similar algorithms 7,[20][21][22][23] , Shrinkwrap solves the phase problem without requiring any a priori knowledge about the object.…”

“…Variations on the principle of flash X-ray imaging demonstrated in this experiment have subsequently been used in a wide range of coherent experiments at FLASH and elsewhere [7,9,12,18,19,33,50]. [9] to study the morphology of aerosols in free flight, Figure 5.…”

“…This provides a convenient real-space representation of the sample at each time step, enabling researchers to directly visualise sample evolution over time. Figure 13 shows real-space reconstructions for the sample studied in [7], where separate diffraction patterns have been indi- Single shot X-ray diffraction patterns of a silicon membrane driven well beyond equilibrium conditions by a visible light drive laser at time delays of -5 ps (corresponding to the object just before the laser excitation pulse) and at 10 ps, 15 ps, 20 ps, 40 ps, and 140 ps after the laser pulse. The increase in speckle intensity at lower frequencies at longer delays combined with loss of high-spatial frequency information quantify the loss of spatial order as the sample ablates and turns into a plasma.…”

Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse

“…It enables creating and probing plasmas, 14,15 hot dense matter, [15][16][17] and warm dense matter, 18,19 as well as the investigation of the interaction of low-fluence ultrafast laser pulses with matter, with applications to structural studies within solidstate physics, 11,[20][21][22][23] nanophysics, 24 molecular physics, and biophysics. 25 The presently operating free-electron lasers can produce laser pulses with durations of a few tens down to a few femtoseconds.…”