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The first atomic X-ray laser


In future, scientists should be able to observe more closely how plants generate sugar from the energy in sunlight or how electricity is generated from solar cells. Researchers at the Center for Free-Electron Laser Science (CFEL) in Hamburg have built the first-ever atomic X-ray laser at the Californian research centre, SLAC. Using neon atoms, they generated ultra-short X-ray bursts with unique colour purity. These laser pulses enable scientists to study charge transfer in photosynthesis at atomic resolution, for example, and potentially replicate the process in technical systems. Physicists can also use the atomic X-ray laser to discover more about the electronic processes in photovoltaic elements. This could ultimately help to develop more efficient solar cells.

Free-electron lasers open up completely new horizons for material scientists, physicists and biologists. This means that, for the first time ever, researchers can use these tools to produce an X-ray laser beam to observe electronic processes or chemical reactions and to decode the structure of proteins, which resists other methods of structure determination. Yet even with an X-ray free-electron laser (XFEL), such as the one used at CFEL - a joint venture between the Max Planck Gesellschaft, the Deutsches Elektronen Synchrotron (DESY) and the University of Hamburg - some investigations remain difficult or even impossible. An atomic X-ray laser can remedy this situation.

"An atomic X-ray laser generates a laser beam with a wavelength that is around 60 times sharper than a free-electron X-ray laser. Moreover, its wavelength remains completely stable, its pulses are shorter and its pulse profile is smoother," explains Nina Rohringer, a scientist in the Max Planck Advanced Study Group at the CFEL. Together with colleagues from the Lawrence Livermore National Laboratory and Colorado State University, the physicist created the atomic X-ray laser.

A free-electron laser pumps the atomic X-ray laser

With its extremely short and monochromatic pulses, this laser can be used not only to observe electronic processes with extremely high temporal and spatial resolution, but also to track how an electron bounces through the relevant molecule during photosynthesis. "This kind of laser also allows us to study non-linear effects," says Nina Rohringer. This means that the optical properties can be manipulated in such a way that light only moves at the speed of an endurance runner: not only an exciting development for physicists, but something that could also prove useful in photonics, in which electronic components are operated using light.

While the atomic X-ray laser has an edge over the free-electron laser in many respects, the latter also offers compelling advantages. The free-electron laser (FEL) emits a more intense laser light and covers a broader wavelength spectrum. Furthermore, the X-ray laser created by Nina Rohringer and her team would not work without a free-electron laser; the researchers need this laser to pump the required energy into neon atoms to produce laser light in the noble gas. Nina Rohringer and her colleagues therefore used the LCLS free-electron laser at the SLAC National Accelerator Laboratory in Menlo Park in California.

Ionised neon atoms emit short X-ray pulses

In free-electron lasers, electrons are accelerated almost to the speed of light using particle accelerators and sent through a structure of strong magnets in a defined orbit. This produces laser-like radiation in the X-ray range. In contrast, traditional optical lasers are based on the radiation of atoms, which are excited to emit light. This emission of light intensifies in the laser medium itself. Previously, this was not possible in the X-ray range because the excitation of the atoms required very intensive radiation in this range. Rohringer's team used the LCLS to implement the first atom-based X-ray laser - more than 40 years after the original idea for such a device was first published.

The researchers sent the LCLS X-ray pulse, which measured 40 to 80 femtoseconds (one femtosecond is one quadrillionth (10-15) of a second), through a cell with very dense neon gas. The X-ray cut a narrow path through the gas, ionising the neon atoms. This means that the X-ray drove out an inner-shell electron from each neon atom. The remaining outer-shell electrons in the atoms soon slipped inwards, causing an X-ray pulse to be emitted.

According to the laser principle of self-amplification, this pulse stimulated the next atom to emit an X-ray pulse with the result that several pulses overlapped to form one X-ray laser burst. The wavelength of this X-ray measured 1.46 nanometres (one nanometre is one millionth of a millimetre). To put this in context, most of the lasers used in the optical range have a wavelength of 800 nanometres. The wavelength determines the size of the details that can still be identified in the corresponding light.

A film camera for chemical reactions

The free-electron X-ray lasers and the atomic X-ray lasers are suitable for a variety of different tasks, but they also work very well together: their X-ray bursts have different wavelengths, creating a two-colour X-ray laser in which both pulses are perfectly synchronised. "We can use this to start a process with one pulse - such as a chemical reaction or an excitement or a structural change in a solid state - and to take a photograph of this process with the pulse of the other colour after a certain period of time," explains Nina Rohringer. If one of the pulses is directed via a precisely defined detour, it can be delayed by a specified short period of time, to take a photograph of various stages of a chemical reaction. As both pulses are generated simultaneously, this period of time can be precisely determined.

Rohringer would now like to develop the atomic X-ray laser further at the Hamburg-based CFEL: "We are investigating, for example, how we can achieve even higher energies and whether it is possible to use molecules, such as oxygen, instead of neon atoms, as a laser medium." This could result in an X-ray laser that generates short pulses of sharply defined wavelengths and in this way covers a broader range of wavelengths. This is a prerequisite for conducting spectroscopic studies, which are only possible using light from a variable wavelength.

Source: Max Planck Society