Methods at BESSY II: Versatile cross-correlator for ultrafast X-ray experiments

Particularly in the soft X-ray range experimentalists are lacking a broadband method to correlate ultrashort X-ray and laser pulses in space and time. Only recently, a team from Helmholtz-Zentrum Berlin and the University of Potsdam was able to achieve this by utilizing a standard molybdenum-silicon (Mo/Si) multilayer mirror at the FemtoSpeX facility at BESSY II. They use femtosecond laser pulses to modulate the multilayer period under the Bragg condition on a sub-picosecond up to nanosecond timescale which in turn strongly affects the mirror’s X-ray reflectivity. The presented Mo/Si cross-correlator works for the soft up to the hard X-ray regime as well as for a broad range of laser pump wavelengths (mid-IR to UV) and renders this technique as an easy to implement and versatile timing tool for various synchrotron- and lab-based pump-probe experiments. The results are published in the journal of "Structural Dynamics".

Accessing the intrinsic timescales of microscopic processes has led to new insights and a deeper understanding of physical, chemical, and biological systems in the last decades. Third-generation synchrotron sources, like BESSY II, offer by default isolated short X-ray pulses of approx. 50ps duration. These short X-ray pulses can be combined with an ultrashort excitation stimulus, such as a femtosecond laser system, for so-called pump-probe experiments. This method can be applied to various experimental techniques such as X-ray scattering, absorption, or electron spectroscopy. At BESSY II the low-α mode provide pulses < 10ps and the FemtoSpeX laser-slicing facility with its 100fs X-ray pulses allows for even higher temporal resolution. In the near future the BESSY VSR mode will come to life yielding X-rays with a few picoseconds pulse duration which will be permanently available at all beamlines for advanced time domain experiments with X-rays.

Finding the overlap

In any time-resolved pump-probe experiment the vital prerequisite is to exactly know the spatial and temporal overlap of the pump and probe pulses on the sample of interest. While spatial overlap can be achieved to some extent by overlapping the two different light beams on a pinhole finding the temporal overlap is generally much more complicated. This problem has already been solved by the laser community several decades ago by using non-linear crystals in order to get temporal overlap of two optical light pulses. Unfortunately, there are no such materials available for correlating optical and X-ray pulses in a similar manner.

Coherent lattice dynamics

The main goal for the team from Helmholtz-Zentrum Berlin and the University of Potsdam was to find a versatile method for cross-correlating a broad range of X-ray and laser photon energies also on the different time scales from 100fs to 100ps as available at BESSY II. They chose a standard molybdenum-silicon (Mo/Si) multilayer mirror as optimized for the soft X-ray regime. Such mirrors consist of alternating metallic molybdenum and semi-conducting silicon layers of nanometer thickness which result in so-called superlattice Bragg peaks that reflect X-rays from approx. 100eV up to the hard X-ray regime with an efficiency of up to 70%.

Using a laser with 50fs long pulses of 800nm wavelength the experimentalists optically excite the Mo/Si mirror which leads to absorption of the light only in the metallic molybdenum layers. This ultrafast heating of only every second layer results in the quasi-instantaneous excitation of coherent acoustic phonons that strongly modulate the reflectivity of the Mo/Si mirror on two different time scales. First, a fast intensity oscillation of the superlattice Bragg peaks of up to 10% amplitude and 600fs oscillation period and second a strong shift of the superlattice Bragg peak on a 10ps time scale with more than 20% transient signal change up to nanosecond delays. As initially desired, both effects allow for easily finding the temporal overlap of X-ray probe and laser pump pulses on a broad range of time scales.

Easy implementation

The presented technique is not only applicable for a broad range of photon energies but also requires no changes to the available sample environment, since the excited dynamics is independent of external fields and temperature and can be even probed under ambient conditions. Additionally, the used Mo/Si mirror is also extremely resistant against laser and/or X-ray as well as oxidation damage. The possibility to change the mirror parameters as well as the deep understanding of the underlying ultrafast structural dynamics in such multilayer mirrors enables to further optimize and adapt this concept for special applications.

Just recently the Mo/Si cross-correlator was successfully used at UE52/SGM within the transmission NEXAFS chamber to precisely determine the temporal overlap of BESSY II’s hybrid bunch and laser pulses from the newly build MHz system.

Publication: "Versatile soft X-ray-optical cross-correlator for ultrafast applications",  Daniel Schick, Sebastian Eckert, Niko Pontius, Rolf Mitzner, Alexander Föhlisch, Karsten Holldack and Florian Sorgenfrei, Structural Dynamics (2016)
DOI: 10.1063/1.4964296