Ultrafast switching dynamics in short pulse laser-driven phase change materials
Final Report Abstract
In summary, time-resolved X-ray scattering using the ultrabright and ultrashort X-ray pulses of the LCLS free electron laser has enabled us to directly monitor the structural pathways during fs laser-induced crystallization of phase change materials. Despite using fs laser pulses, which allow generating states of very strong electronic excitation, our results unambiguously evidence the thermal character of crystallization and amorphization. Interestingly, all investigated materials exhibit very similar dynamics. In particular, for fs excitation crystallization occurs from an intermediate melt-quenched phase thus resembling the crystallization conditions in real PCM-devices. Scince the given sample configuration allows preparing this phase with a well-defined (and by the laser fluence tunable) temperature above the crystallization temperature, a straightforward extension of the time-resolved X-ray scattering experiments discussed here is possible which might allow for example a quantitative determination of both, nucleation and growth rates as a function of temperature. This approach has been used during our 2nd PCM-beamtime at the LCLS in 2014 and the obtained results are still being analyzed. Time-resolved single-pulse X-ray scattering with its capability to probe highly transient states has, therefore, the potential to ultimately settle the controversies about the phase switching mechanisms/pathways and to obtain quantitative information critical to understand and optimize PCMs for future applications. Beyond the study of PCMs, the same experimental scheme appears to be well suited to generate and study in general the structural properties of and phase transitions in metastable, super-cooled states of materials. A corresponding beamtime proposal exploiting this approach has recently been submitted.
