This theme is motivated by large emerging interest in interrogating, visualizing and understanding time-dependent phenomena in materials at the nanoscale at ultra-short (femtosecond to nanosecond) and operando (microsecond to millisecond) timescales. This includes understanding the evolution of metastable materials and lattice phase changes, nonequilibrium mechanical responses, plasmonic processes, and the dynamics of the interaction of materials with various excitation quanta (e.g., optical, magnetic, or electronic) on such timescales.
Limited by the capability of experimental equipment, our window into this world has so far been restricted. However, recent advances in instrumentation are enabling us to change this by probing new contrast mechanisms at greater spectral ranges with high temporal resolution. We aim to offer new tools, methods, and approaches to target central problems in condensed matter physics, novel optical phenomena, and chemical processes, such as catalysis and soft matter. The theme supports and closely affects CNM’s Themes I and II.
Our nanoscale dynamics research rests upon three major experimental approaches:
The first continues to build upon our strong existing efforts in ultrafast optical physics, which we will expand and broaden, for example, into spatially resolved ultrafast spectroscopy and imaging. Characterizing the timescale and channels into which energy flows upon impulsive excitation is fundamental to a wide range of nonequilibrium phenomena. However, discerning mechanical motion such as phonon generation and thermal dissipation can be challenging to evaluate, given that electron-phonon scattering can occur on the femtosecond-to-picosecond timescale. The new approaches that are being developed will be used to address two important areas in the transient response of materials: effects that are triggered inhomogeneously, and the role of electron–phonon dynamics in transient excitations of materials and thermal dissipation. Time-gated photon correlation studies also contribute strongly to Theme I for revealing key nanoscale excitation processes that lead to single photon and correlated photon emission from nanostructures.
The second brings in unique contributions from ultrafast electron microscopy (UEM), which will provides the means to evaluate sample changes spatially (with sub-nanometer resolution) and temporally with regard to real-space local structure, reciprocal space (via electron diffraction), charge distribution, and local electric field on ultrafast timescales. It is a key experimental method that can deliver insights on ultrafast structural and chemical changes to a wide range of systems. There are many areas of nanoscience where the UEM can be highly valuable in advancing our understanding of transient processes, such as in exciton localization, short-lived metastable phases, photo-induced segregation, dynamics in topological materials, plasmonic systems, molecular motors, and magnetic fluctuations, to name a few.
The third is the upgrade of hard X-ray nanoprobe capabilities for time-resolved nanobeam Bragg ptychography in order to fully utilize the upgraded source parameters of the Advanced Photon Source Upgrade and create a unique visualization tool for time-resolved microscopy at high spatial resolution. By synchronizing stroboscopic Scanning X-ray Diffraction Microscopy 3D visualization to 100 ps synchrotron X-ray pulses, our goal is to create a dynamical diffraction four-dimensional ptychography approach that can image strain with nanoscale (~20–30 nm) real-space voxel resolution and 100 ps time resolution. We will be able detect, for example, time-resolved strain induced by acoustic or optical dynamic stimulation of defects in materials. This capability can broadly contribute to the understanding and control of dynamic electron–phonon processes in materials and allow researchers to study the role of defects or inhomogeneities in triggering materials phenomena within large rendered volumes.
These three complementary approaches, empowered by data science, artificial intelligence and computational algorithms, will work closely with one another, and will be integrated with CNM’s other themes.