Observing Subpicosecond Electron Bunches from an Ultracold Source: An Initial Experiment

Identification of new sources that produce electrons faster could aid in the development of the many electron-based imaging techniques. In a recent research paper published in Physical review lettersIn 2008, a team of researchers at Eindhoven University of Technology demonstrated the scattering of sub-picosecond electron beams from an extremely cold electron source.

“Our research group is developing the next generation of ultrafast electron sources to push imaging technologies such as ultrafast electron diffraction to the next level,” Tim de Raat, one of the researchers who conducted the study, told Phys.org.

“The idea of ​​using laser-cooled supercooled gas clouds as an electron source to improve brightness is state-of-the-art It was first presented in a paper published in 2005. Since then, research efforts have produced multiple versions of such a cryogenic electronic source, with the most recent focus (used in this work) on making the source compact, easy to align and operate, and more stable, such as described in another previous paper which also studied the properties of the transversal electron beam.”

The primary goal of the recent work by de Radt and colleagues was to further evaluate the performance of the compressed laser-cooled ultracold source type identified in their previous work, particularly in view of its longitudinal beam characteristics. With a better understanding of the physics behind this source, they can improve its performance and enable its use to develop imaging technologies.

The researchers’ source was created by optically shunting a laser-cooled rubidium gas in a grooved magneto-optical trap via a two-step process. At the self-compressible point of this source, they measured electron beams of up to 735 ± 7 fs (rms).

“We fired a very intense femtosecond laser pulse at the electron beam at the position where the electron group has the shortest length,” de Raat explained. “When the laser pulse hits the electrons, these electrons can scatter out of the group, which is called ‘mind scattering.’ With the electron camera at the end of the beam line, we can see these electrons that have been kicked out of the group as two lines emerging from the group of electrons.”

If the researchers fire the laser pulse at an electron array too soon or too late, they will not hit it and thus fail to see the required electron scattering out. In their experiments, they attempted to determine how long they could scatter these electrons (i.e., measure the length of the electron group), by slowly varying the delay time between firing the laser pulse and the group of electrons. This experiment showed that the electron cluster originating from its source was in the lower picosecond range, which had not been observed before.

“We found that the quality of the longitudinal (emission) beam is not limited by electron temperature, as is the quality of the transverse (emission) beam, but rather by the combination of the ionization process (the way electrons leave atoms) and energy diffusion.”

“Moreover, since it turns out that the ionization process itself takes about a picosecond, there is no need to use a femtosecond ionization laser pulse. We can thus increase the length of the laser ionization pulses by a factor of ten without affecting the electron group length (longitudinal quality), which allows us to use a range of Narrower and more accurate laser wavelength. This opens up a new way to improve cross-sectional beam quality (transmittance).”

Recent work by de Radt and colleagues highlights the value of a compressed ultracold source that they have realized for producing ultrafast electron beams. In addition, having studied the physics and properties of this source further, the team can now predict with high accuracy how short the electron pulses will be. This, in turn, allows them to shorten these pulses at the expense of the energy spread through the source or vice versa.

In the future, the results gathered by this team of researchers could pave the way for the development of high-performance imaging technologies that can advance research in many areas. In their next study, de Radt and colleagues will begin to explore some of the most promising applications of the electron source.

“Now that the physics behind the ultracold electron source are well understood, and the properties have been measured, the source is moving from experimental proof-of-principle to reliable electronic source,” de Raat added.

“This source can be used in many exciting applications, such as single-shot and ultrafast electron crystallography of proteins, which will be revolutionary. As a novel new application, this source would be ideally suited as an injection to accelerate dielectric lasers. Our future studies will therefore focus on applications that cannot be achieved only by using the unique properties of this source.”

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