Two-colour double X-ray laser pulses to study ultrafast processes

A team working at the SACLA X-ray Free-Electron Laser (XFEL) in Japan has succeeded in generating ultrabright two-colour X-ray laser pulses, for the first time in the hard X-ray region. Their work has been published in the journal Nature Communications.

SACLA: in-vacuum variable-gap undulators (about 130 m long).

XFELs are powerful light sources for investigating matter, with various applications in biology, chemistry, physics and materials science. They have the capacity to deliver radiation 10 billion times brighter and with pulses 1000 times shorter than existing synchrotron X-ray radiation sources but, until now, XFELs have normally emitted one radiation pulse at a single wavelength like conventional visible lasers.

The team, led by Dr Toru Hara of the RIKEN SPring-8 Center, has succeeded in creating double X-ray pulses with tuneable wavelengths that can be relatively separated by more than 30%. This was achieved using variable-gap undulators that act as a radiator and whose resonant wavelength can be largely varied by changing the magnetic field strength.

Measured spectrum of the two-colour XFEL.

“The relative separation we have achieved is 10 times bigger than what had been achieved in the past and will make two-colour lasers much easier to use as a light source,” said Dr Hara. “In addition, the two-colour pulses can be emitted on different axes to spatially separate them.”

The laser pulses, that last for less than 10 femtoseconds (10^-15 s) and have peak powers of a few gigawatts, can be generated with time intervals adjusted with attosecond (10^-16 s) precision. The authors say this will enable them to “elucidate X-ray-induced ultrafast transitions of electronic states and structures, which will significantly contribute to the advancement of ultrafast chemistry, plasma physics and astrophysics, and X-ray quantum optics”.

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