# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses
### Compact Ultrafast Low-Energy FED: Metallic Tip as Coherent Electron Sources  
The use of sharp field emission tips triggered by fs-laser pulses [252; 253; 254; 255; 256; 257; 258; 259; 260; 261; 262] provides a unique alternative to reach the required transverse source coherence for the ultrafast dynamical study of large molecular structures. Low-energy fs-electron sources based on field emitters are attracting increasing interest in the community. Emission from an atom wide tip should in principle reach the quantum limit opening opportunities for lensless imaging (or point projection) [263; 264; 265; 266; 267] and coherent diffraction [260; 266]. Figure 22 outlines results obtained by Muller et al. [259] for the study of ultrafast photocurrents in nanowires. The authors implemented a point projection that conferred sub-100 fs temporal and \(\approx\)10 nm spatial resolutions. Muller et al. [259] demonstrated an imaging tool capable of monitoring small electric fields around semiconductor nanowires (NWs) and photocurrents by taking advantage of the high sensitivity of sub-keV fs-electron pulses combined with the large magnification of their point projection source. Figure 22: **Setup for time-resolved low-energy electron point projection mode (PPM). ** (**a**) PPM operating principle: photoelectrons, generated from a nanotip by an ultrashort laser pulse, are accelerated towards the sample positioned several \(\upmu\)m away from the tip. A pump laser pulse, variably delayed from the electron probe, photoexcites the sample. An electrostatic lens is used to switch between divergent and collimated (not shown here) diffraction modes. (**b**–**d**) Point projection microscopy of axially doped NWs. InP NWs (radius 15 nm, length 3.5 mm) with p-i-n axial doping profile and 60-nm i-segment in the center are spanned across 2-\(\upmu\)m holes in a gold substrate. Instead of being a real shadow image of the objects shape, projection images are strongly influenced by local fields surrounding the NW. In addition, a spatial inhomogeneity (marked by the white arrows in (**c**) is observed (**d**). (**e**–**g**) Femtosecond imaging of ultrafast photocurrents. (**e**) Projection image of the same NW as in panel (**b**) recorded in pulsed fs-PPM mode at negative \(t\). Photoexcitation by an ultrashort laser pulse leads to a transient, spatially inhomogeneous change of the projected NW diameter (h, normalized difference plot). Data recorded at 70 eV electron energy; scale bars: 500 nm. Different dynamical behavior and amplitudes of the transient diameter change \(\Delta\mathrm{d}_{\mathrm{NW}}\) are observed for the p- and n-doped segments along the NW (**f**), where an empirical three-exponential function was fitted to the data. Both segments show a fast initial photoinduced effect with 10-90 rise times in the p- and n-segments of 140 and 230 fs, respectively, followed by multi-exponential decay on the fs-to-few picosecond time scale. As \(\Delta\mathrm{d}_{\mathrm{NW}}\) is directly proportional to the transient electric field change, the derivative \(\Delta\mathrm{d}_{\mathrm{NW}}/\mathrm{dt}\) plotted in the inset in (**f**) is a direct measure of the instantaneous photocurrent inside the NW. Surface states cause effective radial doping leading to band bending at the NW surface as sketched in (**g**), where r is the radial coordinate, causing a radial photocurrent of electrons, j, and holes, j\({}_{\mathrm{w}}\) after photoexcitation. This leads to a pump-induced transient shift \(\Delta\mathrm{pump}\) of the energies of the conduction band edge (ECB) and valence band edge (EVB), and hence a shift of the vacuum level (E\({}_{\mathrm{vac}}\)) (red shaded area), compared to the reference level E\({}_{\mathrm{ref}}\) (given by the environment), with the magnitude of the shift depending on the specific band bending and doping level. Figure adapted from Reference [259] with permission from Springer Nature.