Fig. 1. Schematic of ultrafast electron holography

Fig. 2. Schematics of different photoemission mechanisms^{［25，36］}. (a) FE; (b) PFE; (c) MPP; (d) OFE

Fig. 3. Mechanisms of PFE, MPP and ATP. (a) Relationship between photoelectron emission and power of gold tip with curvature radius of 20 nm under laser irradiation where bias voltage of 880 V corresponds to PFE process and bias voltage of 0 V corresponds to MPP process^[32]; (b) relationship between photoelectron emissivity and bias at different optical powers^[32]; (c) PFE photoelectron spectroscopy^[47]; (d) MPP photoelectron spectroscopy^[47]; (e) ATP photoelectron spectroscopy^[33]; (f) photoelectron emissivity-light intensity curve^[36]

Fig. 4. Mechanism of OFE. (a) Schematic of OFE^[49]; (b) OFE energy spectrum and CEP dependence of photocurrent^[49]; (c) simulated flight trajectories of electrons with different initial emission phases and laser center wavelength of 8 mm^[48]; (d) relationship between photoelectron emissivity and analyzer potential at different laser wavelengths ^[48]

Fig. 5. OFE of one dimensional carbon tube. (a) Differential conductance curves of OFE through carbon tube^[50]; (b) photoelectron spectra of OFE through carbon tube^[51]; (c) schematics of OFE through carbon tube and valence band^[52]; (d) CEP dependence of OFE current^[52]

Fig. 6. Field emission and photoemission based on zero-dimension nanostructure. (a) Field emission of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecule deposited on Ag(111) substrate^[54]; (b) double barrier tunneling structure based on PTCDA molecule^[54]; (c) interference patter of PTCDA molecule FE^[54]; (d) illustration of quantum dot modified by apex of tungsten tip^[55]; (e) double barrier tunneling model based on quantum dot^[55]; (f) static field emission spectra of quantum dots^[55]; (g) quantum dot photoelectron emission spectra^[55]

Fig. 7. Low energy electron holography. (a) Schematic of low-energy electron holography and graphene topography and detailed topography reconstructed from hologram^[14]; (b) DNA molecular hologram and its reconstructed topography^[21]; (c) protein molecular hologram and its reconstructed topography^[22]

Fig. 8. Ultrafast low-energy electron point projection imaging and its application in ultrafast charge transport characterization. (a) Schematic of ultrafast low-energy electron point projection imaging^[18]; (b) schematics of p-i-n InP nanowire structure and point projection imaging^[18]; (c) carrier transient response curve in p-i-n InP nanowires^[18]; (d) schematic of laser focusing through propagating plasma focusing based on grating structure and schematic of InP nanowire point projection imaging ^[17]; (e) InP nanowire point projection topography^[17]; (f) photocurrent-based laser autocorrelation interference measurement^[17]

Fig. 9. Application of ultrafast low-energy electron point projection imaging in study of ultrafast charge transport dynamics. (a) Ultrafast point projection imaging of double-hole nanoantenna structure^[29]; (b) schematic of ultrafast characterization of electron energy distribution in nanoantenna structure^[13]; (c) point projection imaging of nanoantenna at zero pulse delay^[13]; (d) transient variation of electron energy at left area of nanoantenna^[13]; (e) transient variation of electron energy at middle area of nanoantenna^[13]

Fig. 10. Ultrafast low-energy electron holography. (a) Experimental principle of photoelectron emission and electron interference^[38]; (b) tungsten tip field emission interference fringes before and after laser excitation^[38]; (c) schematic of ultrafast electron holography^[37]; (d) pulse autocorrelation interferometry based on emission current^[37]; (e) gold tip photoemission interference fringes^[37]; (f) tungsten tip photoemission interference fringes^[37]