In an undulator with permanent magnets of alternating polarization, a wiggling cloud of electrons emits a short, sharp pulse of X-rays. [Image: Tobias Wüstefeld, European XFEL]
Researchers have focused the attosecond pulses from the European X-Ray Free-Electron Laser (XFEL) through total-reflection optics to create record peak intensity levels for such hard X-ray pulses (Optica, doi:10.1364/OPTICA.613284). Scientists hope the peak intensity of roughly 1020 W/cm2 could lead to new studies of atomic and molecular dynamics.
“A few years ago, we demonstrated terawatt-scale hard X-ray attosecond pulses at the European XFEL,” said author Jiawei Yan, Deutsches Elektronen-Synchrotron (DESY), Germany. “But generating powerful pulses is only the first step. For experiments, what really matters is what we can deliver to the sample. With this advance, we can now use attosecond X-rays to explore highly nonlinear interactions between X-rays and matter.”
Long laser, ultrashort pulses
The European XFEL, administered by a 12-nation consortium, stretches out under 3.4 km of land near Hamburg, Germany. At the start of the beamline, superconducting cavities accelerate electrons to energies up to 17.5 GeV with 27,000 pulses per second. The accelerated electrons then whiz through the magnetic fields of specially designed undulators, where self-amplified spontaneous emission creates X-rays with wavelengths of 0.05 to 4.7 nm.
Peak intensity of hard X-ray pulses—measured by dividing the XFEL’s fluence by the pulse duration—is a key metric for generating certain nonlinear effects in experiments.
The multinational team that conducted the latest work was led by Jiawei Yan of DESY in Germany, Jumpei Yamada of the University of Osaka in Japan, and Ichiro Inoue of the University of Tokyo in Japan and the University of Hamburg in Germany.
Record intensity
To reach the record peak intensity, the team took advantage of the European XFEL’s self-chirping operation mode, which boosts the energy of the attosecond pulses. The X-rays then bounced four times off the iridium-coated focusing mirrors of an optical package called the advanced Kirkpatrick-Baez mirror system, developed in Japan for the Linac Coherent Light Source (LCLS), located in the United States. After the mirrors focused the pulses vertically and horizontally, they landed in a focal spot just 180 × 330 nm in size—roughly 100,000 times smaller than the cross-section of a human hair.
For verification of the peak intensity, the researchers blasted a 20-μm-thick sheet of copper foil with the attosecond pulses. The nonlinear interaction between the X-rays and the copper atoms generated amplified spontaneous emission from the atoms—a process by which electrons popped out of their inner shells, leaving holes to be filled by other electrons that emitted their own X-rays, amplifying the signal.
“It is almost magical to see this happening,” said Anders Madsen, group leader of the MID instrument at the European XFEL. “Normal fluorescence goes in all directions, regardless of the direction of the incoming beam. But when the ASE threshold is crossed, you see the light cone starting to condense into the forward direction.” The researchers say that their method could enable detailed studies of how electrons behave in atoms and molecules, and in the long term could even allow the control of electrons during chemical reactions.
