Ultrafast electron microscopy is required to realize the high temporal-spatial resolution characterization of ultrafast dynamic processes. The ultrafast transmission electron microscope (TEM) and the ultrafast scanning electron microscope (SEM) have realized the real space observation of ultrafast dynamic processes at the nanoscale and atomic levels. These processes include atomic diffusion, chemical reaction, phase change, and biological macromolecular dynamics. Ultrafast TEM and ultrafast SEM typically use high-energy (>20 keV) electron beams for imaging. However, owing to the small scattering cross-section of high-energy electrons, the detection sensitivity to the weak local electric or optical field of the sample is low. Therefore, the transient charge transport process is challenging to characterize. In contrast, low-energy electrons with energy less than 500 eV have large scattering cross-section and scattering deflection angle for the weak local electric field in the sample and transient light field on the surface of nanostructures. Moreover, characterization with spatial resolution in the order of 10^-10 m can be achieved via electron holography. Therefore, when combined with ultrafast temporal-resolution technology, the low-energy electron holography is expected to characterize the local electromagnetic field distribution and charge transport at the atomic level.

Point electron source projection microscope (referred to as point projection ) is the simplest device to realize low-energy electron holography. By employing the laser-pumped ultrafast electron source, ultrafast low-energy electron holography can be achieved. Because matching the complex electron optical system is not needed, there is no aberration. The spatial resolution mainly depends on the electron source coherence. Therefore, an ultrafast coherent electron source is required to achieve ultrafast high spatial-resolution holography. In the past few years, many mechanisms to generate ultrafast electrons are discovered, including photon-assisted field emission (PFE), multiphoton photoemission (MPP), above-threshold photoemission (ATP), and optical field emission (OFE). Based on these mechanisms, ultrafast coherent electron sources have been built, and high temporal-spatial resolution holography has been achieved. Therefore, the current research is essential to guide the future development of this field more rationally.

Progress First, the fundamental mechanism of ultrafast electron sources is described in detail, and their coherence is discussed. The ultrafast electron source can be realized via laser pulse-modulated field emission. Photoelectron-emission mechanisms, including PFE, MPP, ATP, and OFE, generate ultrafast electron emission, which can achieve femtosecond or even up to sub-femtosecond temporal resolution. Among these mechanisms, the electrons emitted under the PFE mechanism have a low-energy dispersion equivalent to the static field emission (Table 1), resulting to high coherence. Second, low-dimensional materials, such as one-dimension carbon nanotubes (Fig. 5) and zero-dimension quantum dots, have atomic-scale curvature radii and discrete energy levels induced via quantum confinement effect, enabling the high-coherent emitted electrons. Then, the influence of laser power, laser wavelength, radius of curvature, and discrete energy levels on electron source coherence is summarized, as shown in Table 2. Lastly, the development of static and ultrafast low-energy electron holography is introduced. Furthermore, based on high-coherent tungsten tip electron sources, static holographic imaging of single protein molecule, single DNA molecule, graphene lattice, and sub-nanometer scale charge state can be achieved. Using ultrafast electron sources, the sub-10 fs temporal resolution characterization of the ultrafast dynamic process of photogenerated carriers and nanogap charge transfer in semiconductor nanowires can be realized. However, the spatial resolution of electron holography can only reach several nanometers, as shown in Fig. 10, much larger than that in the static cases. The main reason is that the thermal effect, photon energy mismatch, and strong light field acceleration caused by femtosecond laser weaken the electron source coherence, reducing the spatial resolution of the ultrafast holography.

Conclusions and Prospects In conclusion, further improvement of ultrafast electron source coherence based on traditional metal nanotip is challenging owing to its physical property limitation. In contrast, low-dimensional nanomaterial electron sources, such as electron source based on carbon nanotubes, have atomic-scale emission sites and quantized discrete energy levels, which is an important basis for breaking the coherence bottleneck. Additionally, carbon nanotubes have rich carbon-chiral structures and cutting-edge quantum structures, bringing more regulatory dimensions for the optimal design of high-performance electronic sources. In the future, we will develop the optimal carbon tube emission structure by combining the atomic-scale material design based on the first principle and the bottom-up atomic manufacturing technology. This structure is expected to achieve an ultrafast electron source close to the uncertainty principle limit and promote the development of the atomic resolution of ultrafast electron holographic imaging technology.

Solar energy utilization is crucial for all life on earth. For example, photosynthesis in green plants, which is vital for humans and other living creatures to have access to food, depends on solar energy. Furthermore, solar energy is the cleanest energy source and has various other applications, including solar-electric energy conversion through photovoltaics and solar-chemical energy through photosynthesis and photocatalysis. However, the conversion efficiency is currently low and needs to be improved. Therefore, the study of ultrafast dynamic processes at the atomic scale, such as carrier excitation, photoinduced charge separation, charge transfer, and energy transfer, is crucial for revealing the underlying physical mechanisms of photosynthesis, photovoltaic, and photocatalysis, which is significant for improving the conversion efficiency of solar energy.