In the latest release of journal Ultrafast Science, the developer group of scientists, among others from Center for Free-Electron Laser Science, DESY, presented their first demonstration of an ultrafast electron diffractometer based on a THz-compressed electron source. This solution allows capturing the inner ultrafast dynamics of matter, providing structural information on nonequilibrium states and producing, for example, high-quality diffraction patterns.

An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimate electron beam, that has a precise kinetic energy. It may be classified by the type of electric fields generation – DC or RF.
Over the past years, there has been great interest in achieving sub-100-femtosecond (fs) time resolution with sufficient brightness and repetition rate. Thanks to the constantly improving parameters in this devices, we are able to do the direct observations of the primary physical and chemical processes. But the major challenge for generating short electron bunches is to overcome the inherent space-charge broadening effect and to avoid the synchronization noise. Also such the devices are often large and bulky, due to the radio frequency radiation used to power them (GHz band), whose length sets the size of whole setup.

Schemat układu pomiarowego

Schematic set-up of the Terahertz Ultrafast Electron Diffractometer (STEAM – segmented terahertz electron accelerator and manipulator, PPLN – periodically poled lithium niobate). Credit: DESY, Dongfang Zhang


By using a wave in the terahertz range, it is possible to reduce the size of diffractometer. The DESY team converted a small fraction of the 1030 nm infrared optical beam to 257 nm based on two-stage second harmonic generation. Then the UV pulse is directed onto a gold photocathode generating electron pulses, which are accelerated to53 keV by the dc electric field.
“Ultraviolet pulses for photoemission in the DC gun, multicycle THz pulses to drive the DLW device, single-cycle THz pulses to drive the STEAM device, and optical pump laser pulses for the sample excitation are all created using a single, infrared laser system” –  such a solution ensures perfect synchronization between the exciting laser pulse and the measuring pulse, crucial in this kind ultrafast experiments.

The electron bunches from a conventional DC gun were compressed using multicycle THz-powered dielectrically lined waveguide (DLW), which caused 10,000 electrons/pulse in a duration of 180 fs (FWHM) at a 1 kHz repetition rate. To confirm the effectiveness of their actions, scientists used such a beam to probe the structural dynamics of a 35 nm freestanding, single-crystal silicon, demonstrating high-quality diffraction patterns at improved temporal resolution.
The next planned improvement will be to increase the electron bunch energy, which allows penetration of thicker layers or gas phases and to reduce the duration of pulses.
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Reference: THz-Enhanced DC Ultrafast Electron Diffractometer; Dongfang Zhang, Tobias Kroh, Felix Ritzkowsky, Timm Rohwer, Moein Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas H. Matlis, and Franz X. Kärtner; Ultrafast Science, 2021; DOI: 10.34133/2021/9848526