Low repetition-rate, high-resolution femtosecond transmission electron microscopy

The spatial and energy resolutions of state-of-the-art transmission electron microscopes (TEMs) have surpassed 50 pm and 5 meV. However, with respect to the time domain, even the fastest detectors combined with the brightest sources may only be able to reach the microsecond timescale. Thus, conventional methods are incapable of resolving the myriad fundamental ultrafast (i.e., attosecond to picosecond) atomicscale dynamics. The successful demonstration of femtosecond (fs) laser-based (LB) ultrafast electron microscopy (UEM) nearly 20 years ago provided a means to span this nearly 10-order-of-magnitude temporal gap. While nanometer-picosecond UEM studies of dynamics are now well established, ultrafast angstrom-scale imaging has gone largely unrealized. Further, while instrument development has rightly been an emphasis, and while new modalities and uses of pulsed-beam TEM continue to emerge, the overall chemical and materials application space has been only modestly explored to date. In this Perspective, we argue that these apparent shortfalls can be attributed to a simple lack of data and detail. We speculate that present work and continued growth of the field will ultimately lead to the realization that angstrom-scale fs dynamics can indeed be imaged with minimally modified UEM instrumentation and with repetition rates (f(rep)) below-and perhaps even well below-1 MHz. We further argue that the use of low f(rep), whether for LB UEM or for chopped/bunched beams, significantly expands the accessible application space. This calls for systematically establishing modality-specific limits so that especially promising technologies can be pursued, thus, ultimately facilitating broader adoption as individual instrument capabilities expand. Published under an exclusive license by AIP Publishing.

Automated summary

Current transmission electron microscopes have achieved high spatial and energy resolutions, but are limited in their ability to resolve ultrafast atomic-scale dynamics. Femtosecond laser-based ultrafast electron microscopy (UEM) provides a means to bridge this temporal gap, but has yet to be realized in angstrom-scale imaging. The exploration of chemical and materials applications in this field has been limited, which can be attributed to a lack of data and detail. The use of low repetition rates expands the potential applications of UEM, and further research is needed to establish modality-specific limits. Overall, there is a need for continued development and exploration in this field.