DESY scientists have built a compact electron chamber that can capture the ultra-fast, internal dynamics of matter. The system shoots short clusters of electrons at a sample to take snapshots of its current internal structure. It is the first electron diffractometer of its kind to use terahertz radiation for pulse compression. The team of developers around DESY scientists Dongfang Zhang and Franz Kärtner of the CFEL Free Electron Laser Science Center have validated their ultrafast terahertz-powered electron diffractometer by investigating a silicon sample and present their work in the first issue of the journal Ultrafast Science, a new title from the Science group of scientific journals.
Electron diffraction is a way of investigating the internal structure of matter. However, a direct image of the structure is not obtained. Instead, when electrons collide with or pass through a solid sample, they are systematically deflected by electrons from the solid's internal lattice. From the pattern of this diffraction, recorded in a detector, the structure of the internal lattice of the solid can be calculated. To detect dynamic changes in this internal structure, one must use short beams of sufficiently bright electrons. "The shorter the bunch, the faster the exposure time," explains Zhang, who is now a professor at Shanghai Jiao Tong University. "Typically, ultrafast electron diffraction (UED) uses cluster lengths,
State-of-the-art particle accelerators can routinely produce such short electron clusters with great quality. However, these machines are often large and bulky, in part due to the radio frequency radiation used to power them, which operates in the gigahertz band. The wavelength of the radiation determines the size of the entire apparatus. The DESY team now uses terahertz radiation, with wavelengths about a hundred times shorter. "This basically means that the accelerator components, here a lot of compressors, can also be a hundred times smaller," explains Kärtner, who is also a professor and member of the excellence cluster "CUI: Advanced Imaging of Matter" at the University of Hamburg
For their proof-of-principle study, the scientists fired clusters of about 10,000 electrons each at a silicon crystal that was heated with a short laser pulse. The beams lasted for about 180 femtoseconds and clearly show how the crystal lattice of the silicon sample expands rapidly within a picosecond (trillionth of a second) after the laser hits the crystal. "The behavior of silicon under these circumstances is well known, and our measurements fit expectations perfectly, validating our terahertz device," says Zhang. He calculates that in an optimized setup, electron clusters can be squeezed significantly less than 100 femtoseconds, allowing for even faster snapshots.
In addition to its small size, the terahertz electron diffractometer has another advantage that could be even more important for researchers: "Our system is perfectly synchronized, since we use a single laser for all steps: Generate, manipulate, measure and compress electron clusters, produce terahertz radiation and even heat the sample," explains Kärtner. Timing is key in these kinds of ultra-fast experiments. To monitor for rapid structural changes in a sample of matter such as silicon, researchers often repeat the experiment many times, delaying the measurement pulse a little longer each time. The more precise this delay, the better the result. Usually, there needs to be some kind of synchronization between the exciting laser pulse that starts the experiment and the measurement pulse, in this case the electron cluster. If both the start of the experiment and the electron cluster and its manipulation are activated by the same laser, the timing is intrinsically given.
In a next step, the scientists plan to increase the energy of the electrons. Higher energy means that electrons can penetrate thicker samples. Very low energy electrons were used in the prototype and the silicon sample had to be cut to a thickness of only 35 nanometers (millionths of a millimeter). If another acceleration stage were added, the electrons could have enough energy to penetrate samples 30 times thicker, with a thickness of up to 1 micrometer (thousandth of a millimeter), according to the researchers. For even thicker samples, X-rays are typically used. Although X-ray diffraction is a well-established and highly successful technique, electrons usually do not damage the sample as quickly as X-rays."