# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses
### Ultrabright Hybrid DC-Radio Frequency (RF) FED Setups: Compression of Electron Pulses
Major structural changes are illustrated using different background colours in the central unit cell: flat EDO-TTF moieties have a green background, bent EDO-TTF moieties have a pink background, and PF\({}_{6}\) anions have a cyan background. Green sticks denote the atomic displacements from the initial LT structure to the TIS (left) and the HT-type structure (right); displacements were magnified 10 and 5 times, respectively, for clarity. **(d)** Evolution of phase-transition parameter \(\xi\), in a \((\xi_{\rm F},\xi_{\rm B},\xi_{\rm F})\)-Cartesian configuration space (left) and each of its components as a function of \(t\) in the formation of TIS (right), F, B and P denote flat, bent EDO-TTF, and PF\({}_{6}\) moieties, respectively. Figure adapted from Reference [41] with permission from Springer Nature. Photoinduced metal-like phase of monoclinic VO\({}_{2}\). **(e–e)** Photoinduced time-dependent structural changes: **(e)** ackyground-subtracted FED data from \(t\) = 0 to 20 ps. Vertical lines indicate different groups of reflections that are relevant to monitor the monoclinic (M1) to rutile (R) photoinduced structural transition. **(f)** Overall differences in the diffraction intensities from –0.5 to 20 ps. **(g)** Time-resolved diffraction intensity changes showing fast (\(\sim\)300 fs) and slow (\(\sim\)1.6 ps) dynamics. **(h)** Fluence dependence of the fast and slow signal amplitudes as measured for two selected Bragg reflections; inset, transient transmittance measurements. **(i)** Diffraction difference spectrum for the fast dynamics from –0.5 to 1.5 ps. The change in diffracted intensity is plotted with respect to –1 ps. **(j)** Diffraction intensity difference for the slow dynamics. The change in diffracted intensity from 2 to 10 ps (referenced to 2 ps) is shown. Figure adapted from Reference [189] with permission from Science AAAS. region in Figure 19c, in which electronic and nuclear degrees of freedom are decoupled, suggesting that the insulating properties of VO\({}_{2}\) may arise from Mott physics. Different synchronization methods have been put forward to reduce the timing jitter noise in hybrid ultrabright DC-RF FED setups [242; 243; 244] in order to bring this technology to its maximum potential. An elegant and convenient approach that solved this long-lasting problem has been recently demonstrated by Otto et al. [244] who brought the noise level (long term >10 h) below 50 fs (rms)--limited by the resolution of their streak camera. Their lockbox is based on the use of an ultrabroadband photodiode (12.5 GHz), a low phase-noise CW amplifier, and a bandpass filter that produces the high harmonic signal that drives the RF cavity and acts as a reference in a phase feedback loop, i.e., a phase comparator that feeds from an antenna integrated into the cavity. With the time arrival jitter under long-term control, more exciting results are expected to come from these ultrabright electron sources as Siwick's team plans to commercialize their lockbox. In addition to RF pill lens for electron pulse compression, the use of intense few-cycle THz pulses has recently emerged as a means to control ultrabright electron bunches [245]. Figure 20 illustrates the experimental layout implemented by Kealhofer et al. [245] for THz compression and electron pulse characterization. The authors used butterfly-shaped metal resonators to mediate the interaction between the electrons and the THz fields. The first resonator in the beam path acts as a compressor whereas a second unit serves as a streaking device. It should be noted that the implementation of THz pulses instead of RF cavities nearly eliminates the timing jitter drifts that were found to be as small as 4 fs (rms). Thus, Kealhofer et al. [245] demonstrated a technique that will certainly find applications in the field of ultrafast structural dynamics.