Megahertz-XUV Facility Opens for Use

Researchers have established a novel high-frequency laser facility at the University of Tokyo. The coherent extreme ultraviolet light source can reveal details of biological or physical samples with unprecedented clarity. It also allows for investigation of time-dependent phenomena such as ultrafast chemical reactions. Existing facilities for such investigations necessarily require enormous particle accelerators and are prohibitive to many researchers. This new facility should greatly improve access for a broad range of researchers.

Coherent extreme ultraviolet (XUV) and soft X-ray pulses are especially useful for state-of-the-art investigations into fast-acting phenomena, such as certain chemical reactions or biological processes. But there are some drawbacks to how these beams are made. “Facilities to produce coherent XUV and soft X-rays are huge machines based on particle accelerators,” said Prof Katsumi Midorikawa from the U-Tokyo Institute for Photon Science and Technology and Riken Center for Advanced Photonics. “Given the rarity of these facilities and the expense of running experiments there, it presents a barrier to many who might wish to use them. This is what prompted us to create a new kind of facility that we hope will be far more accessible for a greater number of researchers to use.”

Setup for multiport coherent XUV source: schematic of the mode-locked oscillator (a), of the gain module in laser operation (b), the Brewster plate reflecting HH pulses (c), and photos of the laser system (d; click to enlarge; source: Springer Nature / U. Tokyo)

The new XUV source facility is much, much smaller than any that has come before it. It is housed inside a relatively modest lab underground at the University of Tokyo. The bulk of the machine is a 5-by-2-meter vacuum container housing a 100-meter-long resonator which stores the high-power laser light. At two locations on this coil are pockets of special rare gases that alter characteristics of the passing laser. This results in the two separate beams of XUV and soft X-rays, which are cast onto samples undergoing investigation. Light reflected off the samples is then read by high-speed imaging sensors.

“What is really novel about our approach is that the XUV and soft X-ray pulses are extremely short [610 fs] but occur at very high frequencies, in the region of megahertz,” said Midorikawa. “For perspective, established XUV facilities that use synchrotron radiation pulses also in the megahertz region have longer bursts which are less suitable for resolving dynamic phenomena. And those that use so-called X-ray-free electron laser sources have short pulses, but offer low frequencies of around ten to a hundred hertz. So our facility offers the best of both worlds, with the added benefit of being only a fraction of the size and with far lower operating costs.”

This new XUV source offers ultrashort pulses, useful for probing fast phenomena, and high frequencies, useful for investigating the structure and chemical properties of matter. This is possible, due to the process that creates the pulses as the laser interacts with the gas. It is called high-order harmonic generation and also because of this, the facility is the first of its kind capable of producing multiple XUV and soft X-ray beams.

“I have been working in the field of XUV generation and application for thirty years. Although high-order harmonic generation brought a breakthrough in this field, the generation efficiency and pulse repetition rate were still insufficient for many applications,” said Midorikawa. “When I proposed the idea of this facility to my colleagues, they were instantly interested and we were able to acquire a suitable budget to complete it. We all hope this will open the door to new research from materials scientists, chemists and biologists who can finally access this amazing and powerful investigative tool.” (Source: U. Tokyo)

Further reading: N. Kanda et al.: Opening a new route to multiport coherent XUV sources via intracavity high-order harmonic generation, Light Sci Appl 9, nr 168, online September 24, 2020; DOI: 10.1038/s41377-020-00405-5

Link: RIKEN Center for Advanced Photonics (RAP), Rikagaku Kenkyūjo (RIKEN), Wakō, Saitama, Japan

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