Highlight Lecture (Symp. D01)
Most processes in materials naturally occur in conditions far from equilibrium on short time scales. In most cases due to the resolution limitations of conventional analytical techniques, we are confined to conduct experiments near equilibrium or observe the changes in the material post process. Though insight about material’s behavior can be gained from these observations, much of the coupled and convoluted events of complex processes cannot be observed or well understood. As such, a technique is required that combines both high spatial and temporal resolution to observe nanoscale microstructural features evolving on short timescales. Due to this growing interest and need for studying fast dynamics and transient states in material processes, there has been a recent push to develop and construct time resolved transmission electron microscope instrument the temporal resolution of in-situ TEM observations.
Two principal approaches have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. High time resolution is facilitated through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemssision processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus are complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>106 cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the material’s ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher.
The first part of presentation will explain basic operating principles of the stroboscopic approach and briefly review practical considerations of using high current electron probes and their influence on temporal and spatial resolution. The latter half of the talk will focus on a novel extension and recent instrumentation advancement of the single shot approach, Movie Mode Dynamic Transmission Electron Microscope (MM-DTEM). MM-DTEM enables single-pump/multi-probe operation that comprises two unique technologies, the arbitrary waveform generator (AWG) cathode laser system and a high-speed electrostatic deflector array (a schematic is displayed Figure 1). The AWG cathode drive laser produces a series of laser pulses with user-defined pulse durations and delays and is used to stimulate a defined photoemitted electron pulse train. Each pulse captures an image of the sample at a specific time. The fast-switching electrostatic deflector array located below the sample directs each pulse (image) to a separate patch on a large, high-resolution CCD camera. At the end of the experiment, the entire CCD image is read-out and segmented into a time-ordered series of images, i.e., a movie. These technical improvements provide the ability to track the creation, motion, and interaction of individual defects, phase fronts, and chemical reaction fronts, providing invaluable information of the chemical, microstructural and atomic level features that influence the dynamics and kinetics of rapid material processes. Example applications of MM-DTEM will be presented and how this new technique has provided unprecedented insight into the physics of rapid material processes from their early stages (e.g. nucleation) to completion, giving direct, unambiguous information regarding the dynamics of complex processes will be discussed.