Femtosecond optical imaging of nanostructures using ultrafast ultramicroscopy (Conference Presentation)

Imaging transient dynamics of materials and light-matter interaction at the nanoscale is of great interest to the study of condensed phase dynamics and to the field of nano-photonics. However, optical interrogation of the ultrafast dynamics of nanostructures has not been demonstrated as they are diffraction limited. Furthermore, optical methods are limited due to the low scattering of nanostructures and the strong background reflection. Accordingly, The ultrafast imaging of laser induced nanostructure melting was demonstrated via femtosecond x-ray diffraction imaging which provided relatively high temporal (~10 ps) and high spatial (~10 nm) resolution. However, this technique suffers from the inherent difficulty of using a femtosecond X-ray laser source and the damaging nature of the femtosecond X-ray laser probe required for single-shot imaging. Consequently, X-ray pump-probe imaging was never used to image the re-solidification dynamics of surface structures. On the other hand, cryo-electron microscopy achieved sub-nanometer resolution for single particles, however, it requires experiments to be performed in vacuum with highly specialized and costly instrumentation. Here, we we employ a time-resolved variant of ultramicroscopy that we recently developed to study the ultrafast dynamics of laser ablated surfaces. The technique is non-destructive and allows us to compare the transient image with the initial/final image. Accordingly, we determine the characteristic times for melting and re-solidification of nanostructures using optical wavelengths. We also study the formation and melting of Si nanostructures and image, for the first time, the process of non-thermal melting which occurs on the sub-picosecond time scale.