Fig. 1. Schematic of a photocathode used for ultrafast electron diffraction^［65］. Reprinted from Ref. ［65］， with the permission of AIP Publishing

Fig. 2. Schematic layout of ultrashort microbunch electron source^［75］. Reprinted from Ref. ［75］， with the permission of AIP Publishing

Fig. 3. Schematic of laser plasma wakefield acceleration^［95］. Reprinted from Ref. ［95］

Fig. 4. Localized photoemission from a metal tip

Fig. 5. Generation， compression and characterization of subrelativistic electron pulses by light field^［9］

Fig. 6. Concept and experimental setup of electron pulses compression by THz^［181］

Fig. 7. Concept of electron-beam control by optical field^［184］. An electron beam （blue） is modulated by a single field cycle （red） of a phase-controlled waveform when passing through a metallic membrane （green）. The temporally modulated electron current is directly characterized by real-space streaking induced by a second single-cycle field （red）. Reprinted figure with permission from Ref. ［184］ Copyright （2020） by the American Physical Society

Fig. 8. Layout of the experimental setup for the generation and detection of attosecond electron pulse trains^［185］. The two spatiotemporally separated optical traveling waves-the first for the attosecond electron pulse train generation and the second for its analysis-are generated using two independent Michelson interferometers. Reprinted figure with permission from Ref. ［185］ Copyright （2018） by the American Physical Society

Fig. 9. Direct mapping of attosecond electron dynamics with laser^［191］. As an intense laser pulse is reflected on the plasma mirror， it expels electrons at several narrow specific phase windows of the field. These subcycle attosecond electron pulses then experience an integrated momentum kick as they surf the laser electric field （laser streaking） and form the periodic fringes in the far field

Fig. 10. Set-up of ultrafast low energy electron diffraction in a backscattering geometry^［198］. Ultrashort electron pulses （green） from a nanofabricated electron gun probe the dynamical evolution of the laser-excited surface structure

Fig. 11. Schematic of the MeV ultrafast electron diffraction beam line at SLAC^［33］. Reprinted from Ref. ［33］， with the permission of AIP Publishing

Fig. 12. Schematic of keV ultrafast electron diffraction facility^［204］

Fig. 13. Schematic of 4D ultrafast electron microscopy^［209］. Reprinted with permission from Ref. ［209］.Copyright （2007） American Chemical Society

Fig. 14. Schematic and photo of laser-free 4D ultrafast electron microscopy based on radio-frequency pulser^［236］

Fig. 15. Schematic of scanning ultrafast electron microscopy^［239］. Reprinted with permission from Ref. ［239］. Copyright （2011） American Chemical Society

Fig. 16. Schematic of ultrafast cathodoluminescence^［251］. Reprinted with the permission from Ref. ［251］. Copyright 2013 AIP Publishing LLC