Prospects for Electron Imaging with Ultrafast Time Resolution

Many pivotal aspects of material science, biomechanics, and chemistry would benefit from nanometer imaging with ultrafast time resolution. Here we demonstrate the feasibility of short-pulse electron imaging with t10 nanometer/10 picosecond spatio-temporal resolution, sufficient to characterize phenomena that propagate at the speed of sound in materials (1-10 kilometer/second) without smearing. We outline resolution-degrading effects that occur at high current density followed by strategies to mitigate these effects. Finally, we present a model electron imaging system that achieves 10 nanometer/10 picosecond spatio-temporal resolution

Practical Considerations for High Spatial and Temporal Resolution Dynamic Transmission Electron Microscopy

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10{sup 7} electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10{sup -6} s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed

Electronic and Structural Response of Materials to Fast Intense Laser Pulses

In this chapter we review theoretical and experimental studies of the subject indicated in the title:the response of materials to ultrafast and ultra-intense laser pulses. Our primary emphasis is on the semiconductors GaAs and Si, with some discussion of the fullerene C60. Near the end there is also a brief discussion of certain molecules. The theoretical simulations employ tight-binding electron-ion dynamics (TED), a technique which is fully described in the text. The experiments employ sophisticated techniques that have been developed during the past 20 years, and which are described in papers cited in the text. Comparison of the simulations and experiments shows good agreement in all important respects. In the case of the semiconductors GaAs and Si, there is a nonthermal phase transition as the intensity is varied at fixed pulse duration. For GaAs, the transition corresponds to excitation of about 10 % of the valence electrons to the conduction band. For Si, the threshold intensity is approximately the same, but about 15 % of the electrons are excited. These results are qualitatively understandable, because Si has tighter bonding and a smaller band gap

Emulation of Numerical Models With Over-Specified Basis Functions

<p>Mathematical models are frequently used to explore physical systems, but can be computationally expensive to evaluate. In such settings, an emulator is used as a surrogate. In this work, we propose a basis-function approach for computer model emulation. To combine field observations with a collection of runs from the numerical model, we use the proposed emulator within the Kennedy-O’Hagan framework of model calibration. A novel feature of the approach is the use of an over-specified set of basis functions where number of bases used and their inclusion probabilities are treated as unknown quantities. The new approach is found to have smaller predictive uncertainty and computational efficiency than the standard Gaussian process approach to emulation and calibration. Along with several simulation examples focusing on different model characteristics, we also use the method to analyze a dataset on laboratory experiments related to astrophysics.</p