Pump-probe capabilities at the SPB/SFX instrument of the European XFEL.

Pump-probe experiments at X-ray free-electron laser (XFEL) facilities are a powerful tool for studying dynamics at ultrafast and longer timescales. Observing the dynamics in diverse scientific cases requires optical laser systems with a wide range of wavelength, flexible pulse sequences and different pulse durations, especially in the pump source. Here, the pump-probe instrumentation available for measurements at the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) instrument of the European XFEL is reported. The temporal and spatial stability of this instrumentation is also presented.

Time-resolved serial femtosecond crystallography at the European XFEL.

The European XFEL (EuXFEL) is a 3.4-km long X-ray source, which produces femtosecond, ultrabrilliant and spatially coherent X-ray pulses at megahertz (MHz) repetition rates. This X-ray source has been designed to enable the observation of ultrafast processes with near-atomic spatial resolution. Time-resolved crystallographic investigations on biological macromolecules belong to an important class of experiments that explore fundamental and functional structural displacements in these molecules. Due to the unusual MHz X-ray pulse structure at the EuXFEL, these experiments are challenging. Here, we demonstrate how a biological reaction can be followed on ultrafast timescales at the EuXFEL. We investigate the picosecond time range in the photocycle of photoactive yellow protein (PYP) with MHz X-ray pulse rates. We show that difference electron density maps of excellent quality can be obtained. The results connect the previously explored femtosecond PYP dynamics to timescales accessible at synchrotrons. This opens the door to a wide range of time-resolved studies at the EuXFEL.

Initial observations of the femtosecond timing jitter at the European XFEL.

Intense, ultrashort, and high-repetition-rate X-ray pulses, combined with a femtosecond optical laser, allow pump-probe experiments with fast data acquisition and femtosecond time resolution. However, the relative timing of the X-ray pulses and the optical laser pulses can be controlled only to a level of the intrinsic error of the instrument which, without characterization, limits the time resolution of experiments. This limitation inevitably calls for a precise determination of the relative arrival time, which can be used after measurement for sorting and tagging the experimental data to a much finer resolution than it can be controlled to. The observed root-mean-square timing jitter between the X-ray and the optical laser at the SPB/SFX instrument at European XFEL was 308 fs. This first measurement of timing jitter at the European XFEL provides an important step in realizing ultrafast experiments at this novel X-ray source. A method for determining the change in the complex refractive index of samples is also presented.

MHz data collection of a microcrystalline mixture of different jack bean proteins.

We provide a detailed description of a serial femtosecond crystallography (SFX) dataset collected at the European X-ray free-electron laser facility (EuXFEL). The EuXFEL is the first high repetition rate XFEL delivering MHz X-ray pulse trains at 10 Hz. The short spacing (<1 µs) between pulses requires fast flowing microjets for sample injection and high frame rate detectors. A data set was recorded of a microcrystalline mixture of at least three different jack bean proteins (urease, concanavalin A, concanavalin B). A one megapixel Adaptive Gain Integrating Pixel Detector (AGIPD) was used which has not only a high frame rate but also a large dynamic range. This dataset is publicly available through the Coherent X-ray Imaging Data Bank (CXIDB) as a resource for algorithm development and for data analysis training for prospective XFEL users.

Pump-probe laser system at the FXE and SPB/SFX instruments of the European X-ray Free-Electron Laser Facility.

User operation at the European X-ray Free-Electron Laser Facility started at the SASE1 undulator beamline in fall 2017. The majority of the experiments utilize optical lasers (mostly ultrafast) for pump-probe-type measurements in combination with X-ray pulses. This manuscript describes the purpose-developed pump-probe laser system as installed at SASE1, implemented features and plans for further upgrades.

Megahertz data collection from protein microcrystals at an X-ray free-electron laser.

X-ray free-electron lasers (XFELs) enable novel experiments because of their high peak brilliance and femtosecond pulse duration. However, non-superconducting XFELs offer repetition rates of only 10-120 Hz, placing significant demands on beam time and sample consumption. We describe serial femtosecond crystallography experiments performed at the European XFEL, the first MHz repetition rate XFEL, delivering 1.128 MHz X-ray pulse trains at 10 Hz. Given the short spacing between pulses, damage caused by shock waves launched by one XFEL pulse on sample probed by subsequent pulses is a concern. To investigate this issue, we collected data from lysozyme microcrystals, exposed to a ~15 µm XFEL beam. Under these conditions, data quality is independent of whether the first or subsequent pulses of the train were used for data collection. We also analyzed a mixture of microcrystals of jack bean proteins, from which the structure of native, magnesium-containing concanavalin A was determined.

Superresolved femtosecond laser nanosurgery of cells.

We report on femtosecond nanosurgery of fluorescently labeled structures in cells with a spatially superresolved laser beam. The focal spot width is reduced using phase filtering applied with a programmable phase modulator. A comprehensive statistical analysis of the resulting cuts demonstrates an achievable average resolution enhancement of 30 %.

12 MW peak power from a two-crystal Yb:KYW chirped-pulse oscillator with cavity-dumping.

We report on substantial pulse energy increase in Yb:KYW femtosecond laser oscillators by utilizing multiple laser crystals for an enhanced net-gain at higher pump power. The two-crystal oscillator generates pulse energies of 7 µJ at 1 MHz repetition rate which is, to our knowledge the highest energy ever reported from an Yb-doped tungstate fs-laser oscillator. The external pulse compression yields a pulse duration of 416 fs with a peak power of 12 MW being enough for stable white light generation in YAG.

Single-sweep laser writing of 3D-waveguide devices.

We report on a method to create multiple waveguides simultaneously in 3D in fused silica. A combination of adaptive beam shaping with femtosecond laser writing is used to write two waveguides with changing separation and depth. The method is based on a programmable phase modulator and a dynamic variation of the phase-pattern during the writing process. The depth difference can be dynamically varied by changing a chirp parameter of the applied phase grating pattern. It can be employed in various photonic devices such as couplers, splitters and interferometers. Here we demonstrate splitters with both outputs ending in different depth.

Few-cycle OPCPA system at 143 kHz with more than 1 microJ of pulse energy.

We present an OPCPA system delivering 8.8 fs (3.3 optical cycles) pulses with 1.3 microJ of energy at 143 kHz repetition rate. Pump and seed for the parametric amplification are simultaneously generated by a broadband Ti:sapphire oscillator. The spectral components beyond 1000 nm are separated and amplified in an Yb:YAG thin-disk regenerative amplifier. The pulses are characterized using autocorrelation and SPIDER apparatus. With a pulse peak power of nearly 130 MW, the system is well-suited for future table top strong field experiments.

Sub-10-fs pulses from a MHz-NOPA with pulse energies of 0.4 microJ.

We present a non-collinear optical parametric amplifier (NOPA) delivering sub-10-fs pulses with 420 nJ of pulse energy. The system is driven by microjoule pulses from an Yb:KYW oscillator with cavity-dumping and a subsequent single-stage rod-type fiber amplifier at 1-MHz repetition rate. The ultrabroadband seed is based on stable white-light generation from 420 fs long pulses in a YAG plate.

Multi-µJ, CEP-stabilized, two-cycle pulses from an OPCPA system with up to 500 kHz repetition rate.

We present a two-stage OPCPA system based on a Ti:sapphire seed and a thin-disk regenerative amplifier producing compressed pulse energies of more than 3 µJ and durations of less than 6 fs at a high repetition rate of 143 kHz. In combination with the obtained CEP stability and the repetition rate scalability between 100 and 500 kHz the system forms an ideal tool for high field and phase sensitive spectroscopic experiments.

Double waveguide couplers produced by simultaneous femtosecond writing.

We report on a novel method to create waveguide coupler devices in fused silica by combining the technique of beam shaping with femtosecond laser writing. The method is based on a programmable phase modulator and a dynamic variation of the phase-pattern during the writing process. The major advantage is the possibility to create complex devices in a single sweep by simultaneously writing two or more waveguides with changing separation. The guiding properties and the coupling behavior between the waveguides are investigated.

Pulsing dynamics in Ytterbium based chirped-pulse oscillators.

The properties of passively mode-locked laser oscillators based on Ytterbium doped gain media are studied theoretically along with experimental data. Based on the chirped-pulse approach limitations due to excessive non-linearities are avoided, opening up new routes for energy scaling of mode-locked solid-state oscillators. Predictions about potential future pulse energies are made and possible experimental problems are discussed.

Passively mode-locked Yb:KLu(WO4)2 thin-disk oscillator operated in the positive and negative dispersion regime.

We demonstrate a passively mode-locked femtosecond Yb:KLu(WO(4))(2) thin-disk laser oscillator. Chirped-pulse operation in the positive dispersion regime as well as solitary operation have been realized, and the laser performance of both configurations are compared. In the solitary mode-locking regime the output power exceeds 25 W in a diffraction-limited beam, and pulse durations as short as 440 fs at a 34.7 MHz repetition rate have been generated. For the first time we present a chirped-pulse operation of a thin-disk oscillator that yields a maximum average output power of 9.5 W with a Fourier limit of 450 fs.

Passively mode-locked and cavity-dumped Yb:KY(WO(4))(2) oscillator with positive dispersion.

We demonstrate, what is to our knowledge the first passively mode-locked Ytterbium based solid state high energy laser oscillator operated in the positive dispersion regime. Compared to solitary mode-locking the pulse energy can be increased with even broader spectral bandwidth. With high speed cavity dumping the laser generates 2 muJ-pulses at a 1 MHz repetition rate. The chirped output pulses are compressible down to 420 fs.