Ultrafast atomic-scale processes in nanoscale materials

Program Description

I will describe recent experiments carried out at both LCLS and SSRL probing the first atomic length-scale and femtosecond and picosecond time-scale steps associated with nanoscale transformations.  Two recent experiments will be discussed in detail: In the first I will describe studies of the mechanistic transformation pathways underlying shock-induced and light-induced phase transitions in CdSe and CdS nanocrystals   These measurements demonstrate first experimental evidence of a theoretically predicted but not previously observed intermediate/transition state.  They further show how x-rays enable tomographic visualization of nanoscale shape-change dynamics and large-amplitude reversible strains occurring within semiconductor nanowires on femtosecond time-scales.  In the second I will describe single-cycle terahertz-induced structural responses in nanoscale ferroelectric thin films, including BaTiO3 and BiFeO3, using applied light fields as a means of all-optically biasing these materials on sub-picosecond time-scales, coupled with both x-ray scattering and nonlinear optical probes.  These measurements reveal large amplitude electric-field-induced polarization dynamics, changes in the unit cell structure, and strains.