The many imaging techniques that depend on electrons may develop with the discovery of new sources that produce electrons more quickly. A group of scientists from Eindhoven University of Technology has showed how subpicosecond electron bunches from an ultracold electron source can scatter in an article that was recently published in Physical Review Letters.
“Our research group is working to develop the next generation of ultrafast electron sources to push imaging techniques such as ultrafast electron diffraction to the next level,” Tim de Raadt, one of the researchers who carried out the study, told Phys.org.
“The idea of using laser-cooled ultracold gas clouds as an electron source to improve the state-of-the-art in brightness was first introduced in a paper published in 2005. Since then, research efforts have produced multiple versions of such a ultracold electron source, with the most recent one (used in this work) focusing on making the source compact, easy to align and operate, and being more stable, as described in another past paper that also studied the transverse electron beam properties.”
The main goal of the most recent research by de Raadt and his associates was to evaluate the performance of the kind of tiny laser cooled ultracold source they had previously uncovered, paying particular attention to its longitudinal beam characteristics. They could enhance this source’s performance and enable its usage to advance imaging techniques by better comprehending the physics at work in it.
The two-step photoionization of laser-cooled rubidium gas in a grating magneto-optical trap produced the source used by the researchers. They saw electron bunches as brief as 7357 fs (rms) in this source’s self-compression point.
“We fired a very intense femtosecond laser pulse onto the electron bunch at the position in which the electron bunch has the shortest bunch length,” de Raadt explained. “When the laser pulse hits the electrons, it can scatter them out of the bunch, which is called ‘ponderomotive scattering.’ With the electron camera at the end of the beamline we can see these electrons that have been kicked out of the bunch as two stripes coming out of the electron bunch.”
Researchers would miss an electron bunch if they fired their laser pulse at it too soon or too late, failing to observe the expected outward electron scattering. By gradually altering the delay time between the firing of the laser pulse and the electron bunch, they attempted to ascertain how long they would be able to scatter these electrons (i.e., measuring the length of the electron bunch). This experiment revealed the first-ever observation of an electron bunch coming from its source at a subpicosecond scale.
“We found that the longitudinal beam quality (emittance) is not limited by the electron temperature, as is the case for the transverse beam quality (emittances), but rather by the combination of the ionization process (the way in which the electrons leave the atoms) and the energy spread,” de Raadt said.
“Furthermore, since it turns out the ionization process itself takes about a picosecond, there is no need for us to use a femtosecond ionization laser pulse. We can thus increase the ionization laser pulse length by a factor of ten without impacting the electron bunch length (longitudinal quality), which allows us to use a narrower band and more precise laser wavelength. This opens a new way to improve the transverse beam quality (emittance).”
Recent research by de Raadt and his associates demonstrates the utility of the small ultracold source they developed for generating ultrafast electron bunches. The team is also able to forecast with great precision how brief this source’s electron pulses will be following more research into the physics and characteristics of this source. They can then shorten these pulses at the expense of energy diffusing via the source, or the other way around.
Future research in many domains could benefit from the discoveries made by this team of scientists as they work to build highly effective imaging systems. De Raadt and his associates will begin investigating some of the most potential uses of the electron source in their upcoming investigations.
“Now that the physics behind the ultracold electron source is well understood, and the properties have been measured, the source is moving from an experimental proof of principle to a reliable electron source,” de Raadt added.
“This source can be used for various exciting applications, such as potentially single-shot, ultrafast electron crystallography of proteins, which would be revolutionary. As a new novel application, this source would be ideally suited as injector for dielectric laser acceleration. Our future studies will therefore be focused on applications that are only possible using the unique properties of this source.”
In summary, the first experimental observation of subpicosecond electron bunches from an ultracold source represents a remarkable achievement with far-reaching implications. It not only expands our understanding of ultrafast dynamics but also holds great potential for transformative advancements in various scientific disciplines and technological applications. This breakthrough sets the stage for further exploration, innovation, and discoveries in the exciting realm of ultrafast electron beam science.
