Imaging nanostructure phase transition through ultrafast far-field optical ultramicroscopy

Summary: Imaging photo-induced ultrafast dynamics of nanostructure phase transition is of great interest to the fields of laser-matter interactions and nanotechnology. However, conventional ultrafast far-field optical imaging methods cannot image nanostructures as their scattering scales as D6, with D being the diamater, leading to a vanishing signal-to-noise ratio. Here, we use ultrafast ultramicroscopy to capture the spatiotemporal evolution of surface nanostructures as they undergo melting, spallation, and re-solidification processes. Our experimental observations, combined with finite difference time domain (FDTD) simulations, show agreement with molecular dynamic simulations on ultrashort laser pulse-irradiated metallic nanoparticles and suggest the occurrence of melting of nanostructures followed by photomechanical spallation within a few picoseconds. At longer timescales, we image the re-solidification dynamics of the melted nanostructures occurring within nanoseconds. The re-solidification time for nanostructured surface occurs an order of magnitude faster than for an initially flat surface. Our study demonstrates a simple but powerful far-field optical approach for studying ultrafast dynamics of nanostructures.