Scanning ultrafast electron microscopy: Four-dimensional imaging of materials dynamics in space and time
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Introduction Research in materials science and engineering has become more and more interdisciplinary and continues to evolve and broaden in the 21st century. A few burgeoning areas, such as two-dimensional (2D) materials and hybrid systems,1,2 halide perovskites,3 and metamaterials and metasurfaces,4 have received much attention in recent years, especially for their technological applications. To elucidate carrier dynamics in optoelectronic materials and devices for further control, it is necessary to use time-resolved techniques with appropriate temporal resolutions. This has been seen in the last four decades with femtosecond all-optical spectroscopy methods becoming mature and broadly adopted in studies of a wide variety of materials as well as numerous physical, chemical, and biological systems.5–9 However, limitations in spatial resolution remain because of the fundamental diffraction limit of light and acquisition of averaged results due to ensemble probing, which is the acquisition of signals from a larger region of a sample. To mitigate the latter constraint of sample averaging, optical pump-probe microscopy has been developed to provide diffraction-limited or subdiffraction solutions.10–12 The increasing need to directly image materials dynamics at the atomic level with simultaneous high spatial and
temporal resolutions led to the invention of four-dimensional ultrafast electron microscopy (4D-UEM) in 2005,13 pioneered by Zewail and co-workers at the California Institute of Technology (Caltech) following the development of ultrafast electron diffraction and crystallography.14,15 This novel methodology was initially instrumented using a modified transmission electron microscope (TEM) that allows the entry of two laser beams for photoexcitation of specimens (pump) and generation of ultrashort photoelectron pulses (probe) from a flat LaB6 electron emitter in a pump-probe stroboscopic scheme.13 The fourth dimension of ultrashort time added to the versatile three-dimensional (3D) space- and energy-resolved electron microscopy techniques opens up new possibilities to visualize light‒matter interactions, including photon-induced near-field electron microscopy.16,17 The Introductory,18 Feist et al.,19 and Pomarico et al.20 articles in this issue provide further details for the up-to-date development of this version of UEM. In 2009, D.-S. Yang proposed a scanning electron microscope (SEM) for ultrafast imaging. Instrumentation for the first-generation apparatus was begun in December 2009 by Yang and O.F. Mohammed under Zewail’s supervision at Caltech;21,22 the second-generation apparatus was built at King Abdullah University of Science and Technology by
Ding-Shyue Yang, University of Houston, USA; [email protected] Bolin Liao, University of California, Santa Barbara, USA; [email protected] Omar F. Mohammed, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Saudi Arabia; [email protected] doi:10.1557/mrs.2018.149
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