High spatiotemporal resolution transmission electron microscopy and diffraction: Progress from subnanosecond laser-induced structural dynamics to femtosecond quantum tomography

The use of short photoelectron pulses, pioneered in the 1980s, opened up the possibility of studying structural dynamics with high spatiotemporal resolution. The combination of nano-pico-femtosecond lasers with electron- based technology has become extremely fruitful for observing the behavior of atoms and molecules on their natural length and time scales. In imaging mode, this concept soon led to the creation of 4D transmission electron microscopy. In the electron diffraction mode, the achievement of ultrabright electron sources provided a unique opportunity to shoot molecular movies with atomic resolution. These sources are at their fundamental space charge limit with sufficient brightness to literally light up atomic motions. The high sensitivity of this approach, combined with low radiation damage, made it possible to atomically resolve reaction dynamics with nanograms of material. In contrast to the X-ray free electron lasers (XFELs), the development of ultrabright electron sources made it possible to conduct experiments on very thin films of promising materials in small-scale facilities in standard laboratories. The extension to quantum tomography has recently opened a new page in the study of matter using short electron bunches. Here we review the development of ultrafast transmission electron microscopy and diffraction techniques that enable detection of structural dynamics on the primary timescales.