First lasing of Terahertz free-electron laser at PITZ

New add-on could enhance the research opportunities at X-ray lasers.

The Photo Injector Test Facility at DESY in Zeuthen (PITZ) has reached a major milestone: a free-electron laser (FEL) driven by the photoinjector has generated its first laser light in the Terahertz (THz) wavelength regime. Pulses with a wavelength of about 0.1 millimetres were produced with a repetition rate of up to one Megahertz. The laser is the first high-power Terahertz FEL worldwide working according to the so-called SASE principle, the self-amplification of spontaneous emission. It will now be used to characterise the properties of the intense radiation for a possible future application at the European XFEL. “What started as an idea 10 years ago has now successfully been realized by the team at PITZ and opens up new possibilities for advancing science with THz radiation at the premier hard x-ray FEL and beyond”, says Wim Leemans, Director of the Accelerator Division at DESY.

DESY scientist and project leader Mikhail Krasilnikov says: “The first lasing is a great success, involving many scientists and members of technical groups to set up this experiment. We want to demonstrate the possibility of adding such a THz source to the experimental stations at the European XFEL in Hamburg. This will enable new types of experiments, combining the THz radiation with the hard X-rays produced by the European XFEL.”

Terahertz radiation is gaining more and more interest in the scientific community, because it can start a lot of interesting processes in all kinds of materials, amongst them biological materials. This is because its wavelength between 0.1 and 1 millimetres lies in the area where molecules, e.g. DNA change their rotational or vibrational states when absorbing the radiation. Probing then with high resolution hard X-rays just after these processes have been initiated by the THz pulse can tell a lot about the material properties of newly developed drugs etc. Possible areas of research of this so-called pump-probe experiments cover a wide variety of scientific fields like study of quantum matter, magnetism, complex fluids, petroleum, bio-fluids, membrane proteins, bio-molecules (e.g. dynamics and function of metal ions in bio-systems), condensed matter technologies (glasses, ceramics, catalysts, zeolytes, batteries and fuel cells). “This makes this research of ultimate importance also for direct applications demanded by modern society,” explains Krasilnikov.

THz sources based on optical lasers are around for a while, but so far, they do not have the necessary properties for a lot of interesting experiments. The accelerator-based THz laser at PITZ can provide several properties needed to explore new regions of interest: A high intensity THz source is needed to drive the molecular system far out of equilibrium. In this way, specific properties can be probed with minimal interference by other processes within the material. The wavelength of the laser radiation is narrow and tuneable; this is of high importance to start specific processes as “cleanly” as possible. Besides having a light source which emits only a single wavelength or colour of light it is necessary to be able to tune this colour to match the process of interest. Last but not least a high repetition rate of the produced THz pulses is important to speed up experiments and indeed make some experiments possible in the first place, which would take too long to finish them at facilities with low repetition rates, due to restrictions e.g. in stability.

“Initialized by an idea of Mikhail Yurkov and Evgeny Schneidmiller, two experts in the field of free-electron lasers, these capabilities were envisioned at DESY already more than 10 years ago,“ says Frank Stephan, leader of the PITZ group and co-author of first paper proposing such a facility for the European XFEL. The PITZ group developed a concept of a prototype for an accelerator-based tuneable THz source for pump-probe experiments at the European XFEL. “Important features here were the high simulated pulse energies up to the millijoule range and the pulse time structure,” explains Stephan. “Not only a high repetition rate of one MHz would be able to be demonstrated, but the complete time structure of the THz pulses fits exactly to that of the X-ray pulses produced by the European XFEL since both accelerators driving the FELs produce the enabling electron bunches in the same way.” The tunability of the THz FEL is given just by changing the electron beam energy, which is straight forward at PITZ. These were good arguments for European XFEL to co-finance the project, which is equipped with an undulator on loan from the research centre SLAC (California) from the free-electron laser LCLS-I. Undulators are the special magnet devices which force the accelerated electrons on a zig-zag course, thus causing them to emit high intensity radiation.

Now the PITZ team is eager to characterise in detail the THz laser radiation and optimise the lasing process in order to make it a useful tool for the European XFEL.

“It is a great achievement to see first light on this test instrument in Zeuthen, which has been an ongoing project since 2019,” says Thomas Tschentscher, Scientific Director at European XFEL, “We now have the possibility to further study this route of implementing THz pump sources at the European XFEL, which will be an important addition to increase the breadth of our scientific capabilities.”

Reference

E.A. Schneidmiller, M.V. Yurkov, DESY, Hamburg, Germany, M. Krasilnikov, F. Stephan, DESY, Zeuthen, Germany, “TUNABLE IR/THZ SOURCE FOR PUMP PROBE EXPERIMENTS AT THE EUROPEAN XFEL”, Proceedings of FEL2012, Nara, Japan

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