Observing atomic motions as they occur is the dream goal of ultrafast electron microscopy (UEM). Great progress has been made so far thanks to the efforts of many scientists in developing the photoemission sources and beam blankers needed to create short pulses of electrons for the UEM experiments. While details on these setups have typically been reported, a systematic overview of methods used to obtain a pulsed beam and a comparison of relevant source parameters have not yet been conducted. In this report, we outline the basic requirements and parameters that are important for UEM. Different types of imaging modes in UEM are analyzed and summarized. After reviewing and analyzing the different kinds of photoemission sources and beam blankers that have been reported in the literature, we estimate the reduced brightness for all the photoemission sources reviewed and compare this to the brightness in the continuous and blanked beams. As for the problem of pulse broadening caused by the repulsive forces between electrons, four main methods available to mitigate the dispersion are summarized. We anticipate that the analysis and conclusions provided in this manuscript will be instructive for designing an UEM setup and could thus push the further development of UEM.

Photofield emitters show great potential for many single electron pulsed applications. However, for the brightest pulses >1011A/(m2 sr V), our simulations show that Poisson statistics and stochastic Coulomb interactions limit the brightness and increase the energy spread even with an average of a single electron per pulse. For the systems, we study we find that the energy spread is probably the limiting factor for most applications.

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Emission theory predicts that high brightness cold field emitters can enhance imaging in the electron microscope. This (neglecting chromatic aberration) is because of the large (coherent) probe current available from a high brightness source and is based on theoretically determined values of reduced brightnesses up to 1014 A/(m2 sr V). However, in their analysis, the authors find that statistical Coulomb interactions limit the reduced brightness of even atomically sharp cold field emitters to 1011 A/(m2 sr V) and regular tungsten cold field emitters to around 2×108 A/(m2 sr V). The authors also find that for tip radii in the range from 5 nm to 1 μm, cold field emitters do not outperform larger Schottky (thermal field) emitters. Although this is applied to only one geometry, they expect that similar results will occur for most other cases due to a distinct difference in the behavior of different beam regimes.

It is well known that stochastic Coulomb interactions cause a displacement of electron trajectories in a beam, resulting in an increase of the size of a focused spot. The magnitude of the effect can be calculated by Monte-Carlo simulation, or approximated by various equations.Usually these equations can only be applied in field free sections of an instrument [1]. For an electron source, we would project the displaced beam trajectories back to the virtual source plane and add the broadening to the size of the virtual source. In that way, the reduction of the source brightness can be calculated (for the application of the brightness concept for very small sources see [2]). However, the electrons in the source are accelerated in a non-uniform field. An estimate of the Coulomb effect can still be obtained by cutting the beam in thin slices and adding the effects in the individual slices. For Schottky sources, it has been shown that stochastic interactions indeed have an effect on the maximum obtainable brightness [3]. (To perform a correct calculation, )one needs to know both the total current in the beam and the shape of the beam envelope, since these determine the current density in the slices. These two parameters both depend on the gun configuration: the distance from cathode to anode, the shape of the cathode, the temperature and workfunction of the cathode and the anode extraction voltage. We have recently combined three simulation programs that used to be separate: the calculation of the electric field and electron trajectories in an electron gun, the quantum mechanical calculation of the emission current density from the cathode for the calculated field, and the calculation of the Coulomb effects. So now if we change the extractor voltage, we automatically get a new current and a new beam envelope, which makes the calculation of the Coulomb effects more realistic.We model the gun as shown in figure 1, with a needle that ends in a spherical tip. For a given tip radius, we then calculate the brightness as a function of extraction voltage. An example is given in figure 2. It turns out that for each tip radius, there is a field at which the brightness does not increase anymore, but instead decreases with increasing voltage. The reason for this is that the size of the virtual source increases faster than the current in the beam. Figure 3 plots the maxima found as a function of the tip radius. It also shows the current in the beam at that maximum. Clearly there are two regimes: one for rip radii below 20 nm and one for tip radii above 20 nrn. These can be identified as the pencil beam regime (electrons travel essentially behind each other in the beam) and the Holtzmark regime (electrons have neighbors perpendicular to the axis). The absolute numbers of the maximum brightness are in reasonable agreement with published data [3,4,5,6] and cast doubt on predictions that very sharp tips can reach brightness values of 10 14 A/m 2 srY. It must be noted that our results are only for the configuration of fig. 1 and will cha...