Electron microscopy has become a pivotal tool in numerous fields of study, from materials to biological imaging. While these static images have indeed pushed the frontiers of knowledge, structure-function relationships can only be inferred; integration of the fourth dimension (time) with the three spatial dimensions is necessary for a more rigorous understanding of the forces behind the function. For this reason, we introduced the concept of single electron imaging for the development of four-dimensional ultrafast electron microscopy (UEM). Because single-electron packets have no space-charge broadening, images and diffraction patterns can be observed with atomic-scale spatial resolution and with the time resolution being fundamentally determined by the ultrashort duration of the optical pulse used to generate electrons in the microscope.

The main focus of research is on the study of nonequilibrium structural phase transitions, nanometer-scale phenomena, and biological imaging (see abstracts below). With two microscopes, UEM1 and UEM2, several research areas are being explored, including single-pulse imaging for irreversible structural change. UEM2, Caltech's second generation ultrafast electron microscopy laboratory, has been designed to expand the scope of ultrafast applications by achieving atomic-scale resolution in ultrafast real-space imaging for the first time, and by the added capabilities of electron-energy-loss spectroscopy (EELS), energy-filtered-UEM, and scanning transmission UEM. The same instrument was augmented with two ns lasers extending the mode of operation from single-electron to single-pulse and the time resolution to from fs to s. The first study using UEM2 involved the above features to obtain atomic-scale resolution in real space images, elemental mapping, and electron-energy filtering. We have obtained images of protein crystals with UEM2, and also reported on the direct visualization of embryonic crystallizarion in different materials using the single-pulse mode of UEM2.