Coherence matters

Scientists publish first direct measurements of the coherence properties of LCLS

Free-electron lasers open a fascinating new insight into the structure, ultra fast dynamics and function of matter. With their ultra-short X-ray pulses of only a few femtoseconds and their unprecedented brightness, scientists will be able to make movies at the nano scale, thus getting a clue how friction, catalysis, photosynthesis or other processes of nature work at their fundamental level.

One key parameter for experiments besides duration and brightness of the pulse is the so called coherence of the FEL beam. Perfect pictures of atoms and molecules are obtained only if the waves of the laser light beam are in phase. An international team of scientists led by Dr. Ivan Vartaniants from DESY has now measured the coherence properties of the first hard X-ray laser LCLS at SLAC in California. The researcher group has just published the results of the first direct characterisation of these properties in Physics Review Letters (DOI: 10.1103/PhysRevLett.107.144801).

For their experiments the team of scientists used the well-established double-slit experiment. The FEL light of LCLS is guided to a two pinhole aperture. Traversed light produces a typical interference pattern of the diffracted beams on a screen behind the aperture. By evaluating the shape and contrast of the patterns reveals how well the waves of the laser beam are in phase.

The high intensity of the focussed LCLS beam allows only single pulse exposures for each aperture. Therefore, the researchers worked with a multiple aperture array and exposed a new set of pinholes to every new FEL pulse.

By using varying pinhole separations, the researchers got also information on the maximum size of the X-ray beam that is useful for coherence experiments. The team analysed 110 interference patterns in total using an X-ray wavelength of 1.6 nanometres.

In their publication the scientists conclude that nearly the whole photon beam of the LCLS is usable for coherence experiments, contrary to the small parts, which can only be used for that purpose for experiments at conventional synchrotron radiation sources.