Probing structural and electronic dynamics with ultrafast electron microscopy

In this Perspective, we provide an overview of the field of ultrafast electron microscopy (UEM). We begin by briefly discussing the emergence of methods for probing ultrafast structural dynamics and the information that can be obtained. Distinctions are drawn between the two main types of probes for femtosecond (fs) dynamics - fast electrons and X-ray photons - and emphasis is placed on how the nature of charged particles is exploited in ultrafast electron-based experiments. Following this, we describe the versatility enabled by the ease with which electron trajectories and velocities can be manipulated with transmission electron microscopy (TEM) hardware configurations, and we emphasize how this is translated to the ability to measure scattering intensities in real, reciprocal, and energy space from presurveyed and selected nanoscale volumes. Owing to decades of ongoing research and development into TEM instrumentation combined with advances in specimen holder technology, comprehensive experiments can be conducted on a wide range of materials in various phases via in situ methods. Next, we describe the basic operating concepts of UEM, and we emphasize that its development has led to extension of several of the formidable capabilities of TEM into the fs domain, thus increasing the accessible temporal parameter space by several orders of magnitude. We then divide UEM studies into those conducted in real (imaging), reciprocal (diffraction), and energy (spectroscopy) space. We begin each of these sections by providing a brief description of the basic operating principles and the types of information that can be gathered followed by descriptions of how these approaches are applied in UEM, the type of specimen parameter space that can be probed, and an example of the types of dynamics that can be resolved. We conclude with an Outlook section, wherein we share our perspective on some future directions of the field pertaining to continued instrument development and application of the technique to solving seemingly intractable materials problems in addition to discovery-based research. Our goal with this Perspective is to bring the capabilities of UEM to the attention of materials scientists, chemists, physicists, and engineers in hopes that new avenues of research emerge and to make clear the large parameter space that is opened by extending TEM, and the ability to readily manipulate electron trajectories and energies, into the ultrafast domain.