Ultracold Electron Sources: Exploring Subpicosecond Electron Bunches From Ultracold Sources

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The Advent of Ultracold Electron Sources

In recent years, the pursuit of accelerated electrons for advancing imaging techniques has taken center stage. A research team from Eindhoven University of Technology has made a groundbreaking contribution to this realm with their publication in Physical Review Letters. This paper details their experiment which observed the scattering of subpicosecond electron bunches originating from an ultracold electron source – a novel discovery promising to redefine the landscape of electron-based imaging technologies.

The team, led by Tim de Raadt, is focused on building the next generation of ultrafast electron sources. Their work is expected to propel the field of ultrafast electron diffraction, among other imaging techniques, to new heights. The revolutionary idea of employing laser-cooled ultracold gas clouds as an electron source was first put forward in a 2005 paper. Following that, significant research efforts have been invested to refine the concept of the ultracold electron source. The current version, as utilized in this study, emphasizes on compactness, ease of operation and alignment, and enhanced stability – factors determined by studying the transverse electron beam properties.

the first experimental 3 Ultracold Electron Sources,Subpicosecond Electron Bunches,Advanced Imaging Techniques,Ultrafast Electron Diffraction,Laser-Cooled Ultracold Gas Clouds
A schematic representation of the researchers’ experiment. Credit: de Raadt, Franssen & Luiten. (PRL, 2023)

The Novel Ultracold Source and Its Peculiarities

The primary objective of the researchers was to evaluate the performance of the compact laser-cooled ultracold source, previously identified in their work. They were particularly interested in understanding its longitudinal beam properties, an area that could be optimized to enhance the source’s performance and its applications in advanced imaging techniques.

The team created their source by photoionizing a laser-cooled rubidium gas held in a grating magneto-optical trap using a two-step process. At the self-compression point of this source, they detected electron bunches as short as 735±7 fs (rms). This was achieved through a technique known as ‘ponderomotive scattering,’ where an intense femtosecond laser pulse was fired onto the electron bunch when it was at its shortest length.

Unraveling the Unique Properties of Ultracold Electron Source

De Raadt and his team discovered that if they shot their laser pulse too early or too late at the electron bunch, they failed to generate the desired outward electron scattering. They then aimed to determine the time duration they could scatter these electrons – effectively measuring the length of the electron bunch. This was accomplished by adjusting the delay between firing the laser pulse and the electron bunch. Their findings showed that the electron bunch from their source was of subpicosecond scale, marking a new milestone in this field of research.

The team found that the longitudinal beam quality, or emittance, was determined by the combination of the ionization process and energy spread, not the electron temperature. Moreover, the ionization process, which takes roughly a picosecond, does not necessitate the use of a femtosecond ionization laser pulse. Consequently, they could increase the ionization laser pulse length by a factor of ten without impacting the electron bunch length. This allowed for a more precise laser wavelength, presenting new opportunities to improve the transverse beam quality.

the first experimental 4 Ultracold Electron Sources,Subpicosecond Electron Bunches,Advanced Imaging Techniques,Ultrafast Electron Diffraction,Laser-Cooled Ultracold Gas Clouds
Electrons being scattered out of the main bunch using a intense laser pulse to measure the electron bunch length, Credit: de Raadt, Franssen & Luiten. (PRL, 2023)

Envisioning the Future: Applications and Innovations

The recent work by de Raadt and his team accentuates the importance of the compact ultracold source in generating ultrafast electron bunches. With a deeper understanding of the physics and properties of this source, the team can now accurately predict the length of its electron pulses. This understanding allows them to adjust these pulses, either shortening them at the cost of energy spread or vice versa.

The insights gained from this research could potentially revolutionize imaging techniques across various scientific domains. As they progress, the team plans to explore the most promising applications of this electron source. With the physics of the ultracold electron source now comprehensible and its properties measured, the source is transitioning from an experimental proof of concept to a reliable electron source.

Anticipated applications include single-shot, ultrafast electron crystallography of proteins, a potential game-changer. Additionally, the source could serve as an ideal injector for dielectric laser acceleration. Therefore, the researchers’ future studies will focus on exploring applications that capitalize on the unique properties of the ultracold electron source.

Read Original Article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.205001

Keywords: Ultracold Electron Source, Subpicosecond Electron Bunches, Advanced Imaging Techniques, Ultrafast Electron Diffraction, Laser-Cooled Ultracold Gas Clouds.

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