Development of a compact hard X-ray split-and-delay line for studying ultrafast dynamics at free electron laser sources

The study of condensed matter dynamics on ultrafast timescales is one of the key topics in modern material science research. Hard X-ray free-electron laser sources with extreme peak brightness and ultra short pulses provide excellent conditions for studying ultrafast dynamics in the time domain by employing such techniques as X-ray pump-probe spectroscopy or X-ray photon correlation spectroscopy. However, the intrinsic time structure of FEL sources limits the investigated timescales to 0.2 microseconds or slower. One way of overcoming this limitation is split-and-delay technology. This work presents a new concept for a compact hard X-ray split-and-delay device, enabling such experiments at X-ray FEL sources. The device is designed to split a single X-ray pulse into two fractions introducing time delays from -5 to 815 ps. Accessing such timescales allows to push studies of ultrafast dynamics beyond the intrinsic temporal limit of the X-ray source. The split-and-delay unit is based on Bragg optics and modern technologies for mechanics. Having a compact portable design with dimensions of 60x60x30 cm and a weight of about 60 kg allows to install the device in basically any experimental hutch of a FEL source. The split-and-delay line utilizes a combination of various silicon Bragg optics, arranged in various configurations, enabling the operation in the energy range from 7 to 16 keV. The quality of the beam splitting optics is checked by X-ray topography measurements. A novel method for the split-and-delay line alignment and time delay calibration using a infrared laser setup is developed and successfully used. The infrared setup allows a temporal pre-alignment with a precision better than 22 ps without the need for X-rays. The performance of the split-and-delay setup is checked by measuring the throughput and the delay times with the use of Si(111), Si(220) and Si(422) optics at 7 keV and 9 keV photon energies. Delay times are measured, ranging from 130 ps to 716 ps. The average uncertainty of measured delay times is 16.2 ps. The results show, that ultrafast pump-probe or XPCS experiments can be carried out with the compact split-and-delay line

Compact hard X-ray split-and-delay line for studying ultrafast dynamics at free-electron laser sources

A compact hard X-ray split-and-delay line for studying ultrafast dynamics at free-electron laser sources is presented. The device is capable of splitting a single X-ray pulse into two fractions to introduce time delays from −5 to 815 ps with femtosecond resolution. The split-and-delay line can operate in a wide and continuous energy range between 7 and 16 keV. Compact dimensions of 60 × 60 × 30 cm with a total weight of about 60 kg make it portable and suitable for direct installation in an experimental hutch. The concept of the device is based on crystal diffraction. The piezo-driven stages utilized in the device give nanometre positioning accuracy. On-line monitoring systems based on X-ray cameras and intensity monitors are implemented to provide active alignment feedback. Performance estimates of the system are also presented

Spatial and temporal pre-alignment of an X-ray split-and-delay unit by laser light interferometry

We present a novel experimental setup for performing a precise pre-alignment of a hard X-ray split-and-delay unit based on low coherence light interferometry and high-precision penta-prisms. A split-and-delay unit is a sophisticated perfect crystal-optics device that splits an incoming X-ray pulse into two sub-pulses and generates a controlled time-delay between them. While the availability of a split-and-delay system will make ultrafast time-correlation and X-ray pump-probe experiments possible at free-electron lasers, its alignment process can be very tedious and time-consuming due to its complex construction. By implementing our experimental setup at beamline P10 of PETRA III, we were able to reduce the time of alignment to less than 3 h. We also propose an alternate method for finding the zero-time delay crossing without the use of X-rays or pulsed laser sources. The successful demonstration of this method brings prospect for operating the split-and-delay systems under alignment-time-critical environments such as X-ray free electron laser facilities

Single and multi-pulse based X-ray Photon Correlation Spectroscopy

The ability of pulsed nature of synchrotron radiation opens up the possibility of studying microsecond dynamics in complex materials via speckle-based techniques. Here, we present the study of measuring the dynamics of a colloidal system by combining single and multiple X-ray pulses of a storage ring. In addition, we apply speckle correlation techniques at various pulse patterns to collect correlation functions from nanoseconds to milliseconds. The obtained sample dynamics from all correlation techniques at different pulse patterns are in very good agreement with the expected dynamics of Brownian motions of silica nanoparticles in water. Our study will pave the way for future pulsed X-ray investigations at various synchrotron X-ray sources using individual X-ray pulse patterns

Mapping the 3D position of battery cathode particles in Bragg Coherent Diffractive Imaging

In Bragg coherent diffractive imaging, the precise location of the measured crystals in the interior of the sample is usually missing. Obtaining this information would help the study of the spatially dependent behavior of particles in the bulk of inhomogeneous samples, such as extra-thick battery cathodes. This work presents an approach to determine the 3D position of particles by precisely aligning them at the instrument axis of rotation. In the test experiment reported here, with a 60 µm-thick LiNi0.5Mn1.5O4 battery cathode, the particles were located with a precision of 20 µm in the out-of-plane direction, and the in-plane coordinates were determined with a precision of 1 µm

Double-pulse speckle contrast correlations with near Fourier transform limited free-electron laser light using hard split-and-delay

The ability to deliver two coherent X-ray pulses with precise time-delays ranging from a few femtoseconds to nanoseconds enables critical capabilities of probing ultra-fast phenomena in condensed matter systems at X-ray free electron laser (FEL) sources. Recent progress made in the hard X-ray split-and-delay optics developments now brings a very promising prospect for resolving atomic-scale motions that were not accessible by previous time-resolved techniques. Here, we report on characterizing the spatial and temporal coherence properties of the hard X-ray FEL beam after propagating through split-and-delay optics. Speckle contrast analysis of small-angle scattering measurements from nanoparticles reveals well-preserved transverse coherence of the beam. Measuring intensity fluctuations from successive X-ray pulses also reveals that only single or double temporal modes remain in the transmitted beam, corresponding to nearly Fourier transform limited pulses

Nanosecond X-ray photon correlation spectroscopy using pulse time structure of a storage-ring source

X-ray photon correlation spectroscopy (XPCS) is a routine technique to study slow dynamics in complex systems at storage-ring sources. Achieving nanosecond time resolution with the conventional XPCS technique is, however, still an experimentally challenging task requiring fast detectors and sufficient photon flux. Here, the result of a nanosecond XPCS study of fast colloidal dynamics is shown by employing an adaptive gain integrating pixel detector (AGIPD) operated at frame rates of the intrinsic pulse structure of the storage ring. Correlation functions from single-pulse speckle patterns with the shortest correlation time of 192 ns have been calculated. These studies provide an important step towards routine fast XPCS studies at storage rings

Terahertz Radiation Driven Dynamics of Magnetic DomainStructures Probed by Free-Electron Laser Light

Controlling magnetism on ultra-fast time scales and on nanometer lengthscales is a challenge for modern research in magnetism. Means for inducingdynamics on these time scales are femtosecond optical lasers [1] and THzsources [2]. Probing the dynamics on a nanometer length scale is possiblewith free-electron laser sources. We report on a THz-pump–XUV-probescattering experiment on (Co/Pt)n multilayers (n = 8,16) with perpendicularmagnetic anisotropy (PMA) exhibiting a maze domain pattern. An additionalelectromagnetic undulator available at FLASH was used to produce10-cycle linearly polarized THz pulses. The fundamental wavelength wasset to 150 μm and higher harmonics down to 30 μm have been used as apump. The resulting dynamics have been probed on femtosecond time scales[3] by resonant magnetic small-angle scattering at the cobalt M3 edge. For amultilayer with 8-fold repetition we observed that after 200 fs the scatteringintensity is drastically decreased by one order of magnitude (blue curve inFig. 1(a)). This change is fast compared to the duration of the THz pumppulse, which is about 6 ps long. Besides, a shift of the scattering peak positionto lower Q-values by 14% occurred (red curve in Fig. 1(a)). The latteris similar to what was found when using NIR pumping [4]. However, herewe observed an onset of the peak shift delayed by about 100 fs with respectto the reduction in scattering intensity and a different shape of both signals.Such subtle differences were impossible to resolve in the previous experimentsusing NIR-pump pulses due to the larger temporal jitter (> 100fs).Interestingly, the response is found to be much weaker (scattering intensity)or not resolvable (peak shift) in case of a Co/Pt multilayer with 16-foldrepetition (Fig. 1(b)). The major difference is the PMA of both samples.While the 16-fold multilayer has a strong PMA (K1,eff = 200kJ/m3) the 8-foldmultilayer has an almost vanishing PMA (K1,eff = 30kJ/m3), so that it is muchmore susceptible to magnetic fields and hence the THz magnetic field cancause a significant tilting of the magnetization.[1] E. Beaurepaire, J. Merle, A. Daunois, and J. Bigot, Phys. Rev. Lett. 76,4250 (1996). [2] C. Vicario, C. Ruchert, F. Ardana-Lamas, P.M. Derlet, etal. Nat. Photon. 7, 720 (2013). [3] F. Tavella, N. Stojanovic, G. Geloni, andM. Gensch, Nat Photon. 5, 162 (2011). [4] B. Pfau, S. Schaffert, L. Müller,C. Gutt, A. Al-Shemmary, et al., Nat. Commun. 3, 1100 (2012)

Ultrafast Magnetisation Dynamics at the Low-Fluence Limit Supported by External Magnetic Fields

We report on ultrafast magnetisation dynamics in ferromagnetic cobalt/platinum multilayers upon pumping by near and mid to far infrared radiation, utilizing sub-100 femtosecond free-electron laser pulses. The evolution of the excited magnetic state is studied on femtosecond timescales with nanometre spatial resolution and element selectivity, employing time-resolved magnetic small-angle X-ray scattering. The obtained results contribute to the ongoing discussion to what extent either coupling of the electromagnetic field or rather quasi-instantaneous heating of the electron-system is the driving force for phenomena like ultrafast demagnetization or all-optical helicity-dependent switching