Numerous electron-based imaging methods could be improved by locating new sources that produce electrons more quickly. In a new paper distributed in Actual Survey Letters, a group of specialists at Eindhoven College of Innovation showed the dispersal of subpicosecond electron packs from an ultracold electron source.
Tim de Raadt, one of the study’s authors, told Phys.org, “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.”
“In a 2005 paper, the concept of using laser-cooled ultracold gas clouds as an electron source to improve the current state of brightness was first presented. Since then, numerous versions of such an ultracold electron source have been developed, with the most recent one used in this work focusing on making the source smaller, simpler to align and use, and more stable, as described in a previous paper that also investigated the properties of the transverse electron beam.”
“The next generation of ultrafast electron sources is being developed by our research team in order to advance imaging techniques like ultrafast electron diffraction.”
Tim de Raadt, one of the researchers who carried out the study,
The longitudinal beam properties of the compact laser-cooled ultracold source that de Raadt and his colleagues had previously identified were the primary focus of their most recent work, which aimed to further evaluate its performance. They could improve this source’s performance and make it possible to use it to advance imaging techniques by gaining a better understanding of its physics.
A schematic portrayal of the scientists’ trial Credit: de Raadt, Franssen, and Luiten (PRL, 2023)
The specialists’ source was made by photoionizing laser-cooled rubidium gas in a grinding magneto-optical snare through a two-step process. They measured electron bunches as short as 7357 fs (rms) in this source’s self-compression point.
De Raadt provided the following explanation: “We fired a very intense femtosecond laser pulse onto the electron bunch at the position where the electron bunch has the shortest bunch length.” Ponderomotive scattering occurs when the laser pulse hits the electrons and separates them from the group. These kicked-out electrons can be seen as two stripes emerging from the electron bunch when the electron camera is positioned at the end of the beamline.
On the off chance that the scientists shot their laser beam onto an electron bundle too early or past the point of no return, they wouldn’t hit it and would hence neglect to see the ideal outward electron dissipating. By gradually altering the delay time between the firing of the laser pulse and the electron bunch, they tried to determine how long they would be able to scatter these electrons (i.e., measure the length of the bunch). This experiment demonstrated for the first time that the electron bunch that came from their source was in the subpicosecond range.
Using a powerful laser pulse to measure the length of the electron bunch, electrons are being dispersed from the main group. Credit: Franssen, de Raadt, and Luiten (PRL, 2023)
“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 stated. “This is in contrast to the case for the transverse beam quality (emittances).”
Furthermore, a femtosecond ionization laser pulse is unnecessary because it turns out that the ionization process itself takes only a picosecond. We can subsequently expand the ionization laser beat length by a component of ten without influencing the electron bundle length (longitudinal quality), which permits us to utilize a smaller band and a more exact laser frequency. The transverse beam quality (emittance) can be improved in a novel way as a result of this.
The new work by de Raadt and his associates features the value of the conservative ultracold source they understood for creating ultrafast electron packs. In addition, the team is now able to accurately predict the duration of this source’s electron pulses after further investigation of its physics and characteristics. This thusly permits them to abbreviate these heartbeats to the detriment of energy spreading through the source, or the other way around.
Later on, the discoveries assembled by this group of specialists could prepare for the improvement of profoundly performing imaging strategies that could propel research in various fields. De Raadt and his colleagues will begin investigating some of the electron source’s most promising applications in their subsequent research.
“The source is moving from an experimental proof of principle to a reliable electron source now that the physics behind the ultracold electron source is well understood and the properties have been measured,” de Raadt added.
“This source can be utilized for different invigorating applications, like possibly single-shot, ultrafast electron crystallography of proteins, which would be progressive. As another original application, this source would obviously fit as an injector for dielectric laser speed increases. As a result, the applications that can only be made possible by utilizing this source’s unique characteristics will be the focus of our subsequent research.
More information: T. C. H. de Raadt et al, Subpicosecond ultracold electron source. Physical Review Letters(2023). DOI: 10.1103/PhysRevLett.130.205001.
