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Free electron vortices (FEVs) generated by multiphoton ionization (MPI) with ultrashort laser pulses have attracted significant attention due to their varied symmetries and unusual topological properties. We study two physical mechanisms of coherent control in atomic MPI with bichromatic polarization-shaped femtosecond laser pulses which give rise to the rich variety of FEVs. In the experiments, we combine pulse shaping of a carrier-envelope phase-stable supercontinuum with photoelectron tomography to generate and reconstruct three-dimensional photoelectron momentum distributions (PMDs). Simultaneous measurements of energetically separated photoelectrons from intraband and interband interference in a single PMD allow us to compare phase and polarization control of the angular distributions by both mechanisms. We investigate phase control in three scenarios: first, counterrotating circularly polarized pulses are employed to contrast the phase-insensitive angular momentum eigenstate created by intraband interference via frequency mixing with the phase-sensitive c 7 rotationally symmetric FEV from pure interband interference of two single-color ionization pathways. In the second scenario, we use orthogonal linearly polarized pulses to compare the phase control properties of a six-lobed angular momentum wave packet from intraband interference to those of a complex shaped interband PMD in the presence of phase fluctuations. Finally, we demonstrate phase control of a photoelectron hologram from mixed interband interference. In a (3 + 1) resonance enhanced MPI scheme, the red pump pulse induces a bound electron wave packet probed by the time-delayed blue pulse. The latter simultaneously creates a reference wave packet by three-photon ionization to form the photoelectron hologram. Rotation of the hologram with c 1 or c 5 rotational symmetry maps the time evolution of the bound wave packet. To analyze our results, we develop analytical expressions for the wave functions of intraband and interband interference in perturbative non-resonant MPI. The experiments are complemented with two-dimensional TDSE simulations to follow the FEV formation dynamics and to validate the physical pictures.

We present a novel concept for the generation of ultrashort polarization-shaped bichromatic laser fields. The scheme utilizes a 4f polarization pulse shaper based on a liquid crystal spatial light ...

Fluctuation X-ray scattering (FXS) is an emerging experimental technique in which solution scattering data are collected using X-ray exposures below rotational diffusion times, resulting in angularly anisotropic X-ray snapshots that provide several orders of magnitude more information than traditional solution scattering data. Such experiments can be performed using the ultrashort X-ray pulses provided by a free-electron laser source, allowing one to collect a large number of diffraction patterns in a relatively short time. Here, we describe a test data set for FXS, obtained at the Linac Coherent Light Source, consisting of close to 100 000 multi-particle diffraction patterns originating from approximately 50 to 200 Paramecium Bursaria Chlorella virus particles per snapshot. In addition to the raw data, a selection of high-quality pre-processed diffraction patterns and a reference SAXS profile are provided.

Long wavelength vibrational modes in the ferromagnetic semiconductor ${\mathrm{Ga}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{As}$ are investigated using time resolved x-ray diffraction. At room temperature, we measure oscillations in the x-ray diffraction intensity corresponding to coherent vibrational modes with well-defined wavelengths. When the correlation of magnetic impurities sets in, we observe the transition of the lattice into a disordered state that does not support coherent modes at large wavelengths. Our measurements point toward a magnetically induced broadening of long wavelength vibrational modes in momentum space and their quasilocalization in the real space. More specifically, long wavelength vibrational modes cannot be assigned to a single wavelength but rather should be represented as a superposition of plane waves with different wavelengths. Our findings have strong implications for the phonon-related processes, especially carrier-phonon and phonon-phonon scattering, which govern the electrical conductivity and thermal management of semiconductor-based devices.

We report on the creation and manipulation of three-dimensional (3D) electron vortices from femtosecond multiphoton ionization of atoms. Vortex-shaped photoelectron momentum distributions arise from the superposition of two time-delayed free-electron wave packets with different magnetic quantum numbers. In the experiment, pairs of time-delayed counter-rotating circularly polarized (CRCP) femtosecond laser pulses, generated from a polarization-shaped supercontinuum source, are used to ionize potassium atoms. The resulting 3D electron vortices are reconstructed tomographically from a set of velocity map imaging measurements. By variation of the time delay, the helicity, and the spectral bandwidth of the CRCP pulse sequence, we control the radial vortex shape. Absorption of another photon in the continuum changes the ${c}_{6}$ azimuthal symmetry of the threshold vortex into ${c}_{8}$ for above-threshold ionization. Electron vortices from nonperturbative excitation show ${c}_{4}$ azimuthal symmetry and a $\ensuremath{\pi}$-phase jump in the polar direction. Determination of the relative phase of the superposition state allows us to reconstruct a free-electron wave function.

We apply ultrafast polarization shaping to an ultrabroadband carrier envelope phase (CEP) stable white light supercontinuum to generate polarization-tailored bichromatic laser fields of low-order frequency ratio. The generation of orthogonal linearly and counter-rotating circularly polarized bichromatic fields is achieved by introducing a composite polarizer in the Fourier plane of a 4 f polarization shaper. The resulting Lissajous- and propeller-type polarization profiles are characterized experimentally by cross-correlation trajectories. The scheme provides full control over all bichromatic parameters and allows for individual spectral phase modulation of both colors. Shaper-based CEP control and the generation of tailored bichromatic fields is demonstrated. These bichromatic CEP-stable polarization-shaped ultrashort laser pulses provide a versatile class of waveforms for coherent control experiments.

Multiphoton ionization of potassium atoms with a sequence of two counter-rotating circularly polarized femtosecond laser pulses produces vortex-shaped photoelectron momentum distributions in the polarization plane describing Archimedean spirals. The pulse sequences are produced by polarization shaping and the three-dimensional photoelectron distributions are tomographically reconstructed from velocity map imaging measurements. We show that perturbative ionization leads to electron vortices with c_{6} rotational symmetry. A change from c_{6} to c_{4} rotational symmetry of the vortices is demonstrated for nonperturbative interaction.

X-raying superfluid helium droplets When physicists rotate the superfluid 4 He, it develops a regular array of tiny whirlpools, called vortices. The same phenomenon should occur in helium droplets half a micrometer in size, but studying individual droplets is tricky. Gomez et al. used x-ray diffraction to deduce the shape of individual rotating droplets and image the resulting vortex patterns, which confirmed the superfluidity of the droplets. They found that superfluid droplets can host a surprising number of vortices and can rotate faster than normal droplets without disintegrating. Science , this issue p. 906

Scientific data 3, 160058 (2016). doi:10.1038/sdata.2016.58, Structural studies on living cells by conventional methods are limited to low resolution because radiation damage kills cells long before the necessary dose for high resolution can be delivered. X-ray free-electron lasers circumvent this problem by outrunning key damage processes with an ultra-short and extremely bright coherent X-ray pulse. Diffraction-before-destruction experiments provide high-resolution data from cells that are alive when the femtosecond X-ray pulse traverses the sample. This paper presents two data sets from micron-sized cyanobacteria obtained at the Linac Coherent Light Source, containing a total of 199,000 diffraction patterns. Utilizing this type of diffraction data will require the development of new analysis methods and algorithms for studying structure and structural variability in large populations of cells and to create abstract models. Such studies will allow us to understand living cells and populations of cells in new ways. New X-ray lasers, like the European XFEL, will produce billions of pulses per day, and could open new areas in structural sciences., Published by Nature Publ. Group, London

Single He nanodroplets doped with Xe atoms are studied via ultrafast coherent x-ray diffraction imaging. The diffraction images show that rotating He nanodroplets about 200 nm in diameter contain a small number of symmetrically arranged quantum vortices decorated with Xe clusters. Unexpected large distances of the vortices from the droplet center (≈0.7–0.8 droplet radii) are explained by a significant contribution of the Xe dopants to the total angular momentum of the droplets and a stabilization of widely spaced vortex configurations by the trapped Xe clusters.

It is well-known that the initiation and propagation behaviour of small cracks is strongly influenced by the microstructure. In addition to the visible structure on the surface, in-depth information is essential for the evaluation of influencing factors. Investigations on micro-crack behaviour have been performed with intermetallic γ-based TiAl. Starter notches facilitate systematic studies as cracks initiate from notch tips. In this case artificial notches in the scale less than colony dimensions were prepared by femtosecond pulsed laser radiation, which causes no significant damage. After multistage tensile compression tests under increasing load, several small cracks were generated and analyzed in the SEM. Using femtosecond laser-induced breakdown spectroscopy (fs-LIBS) it was possible to analyze spectrochemically the surrounding microstructure of these micro-cracks and their propagation on the surface.

An overview on femtosecond laser pulse shaping techniques applied to control of the initial photo-physical steps involved in materials processing is presented. First, pulse shaping methodology in frequency domain is introduced and examples of shaped pulses relevant to laser microfabrication are discussed. Then, the use of tailored femtosecond pulses to control the initial steps of laser processing of high band gap materials is demonstrated. In particular, control on basic ionization proc-esses acting as the initial photo-physical step of the ablation dynamics is exerted by highly asymmetric femtosecond laser pulse shapes generated by Third Order Dispersion (TOD).

Laser material processing of dielectrics with temporally asymmetric femtosecond laser pulses of identical fluence, spectrum, and statistical pulse duration is investigated experimentally. To that end single shot structures at the surface of fused silica as a function of fluence and pulse shape are analyzed with the help of scanning electron microscopy. Structures for the bandwidth limited pulses show the known expansion in structure size with increasing laser fluence approaching the diffraction limit, which is 1.4 μm for the 0.5NA microscope objective used. In contrast, structures from the asymmetric pulses are remarkably stable with respect to variations in laser fluence and stay below 300 nm despite doubling the fluence. Different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart are attributed to control of different ionization processes.

There exists a conspicuous gap of knowledge about the organization of life at mesoscopic levels. Ultra-fast coherent diffractive imaging with X-ray free-electron lasers can probe structures at the relevant length scales and may reach sub-nanometer resolution on micron-sized living cells. Here we show that we can introduce a beam of aerosolised cyanobacteria into the focus of the Linac Coherent Light Source and record diffraction patterns from individual living cells at very low noise levels and at high hit ratios. We obtain two-dimensional projection images directly from the diffraction patterns, and present the results as synthetic X-ray Nomarski images calculated from the complex-valued reconstructions. We further demonstrate that it is possible to record diffraction data to nanometer resolution on live cells with X-ray lasers. Extension to sub-nanometer resolution is within reach, although improvements in pulse parameters and X-ray area detectors will be necessary to unlock this potential.

Since small crack propagation behavior is strongly affected by microstructure, very small artificial notches with a length in the submillimeter range are needed for a systematic study of microcrack behavior. Laser processing technique with ultrashort pulses is a micromachining tool which will not cause any serious mechanical damage in metallic materials. Small artificial starter notches were manufactured in medium carbon steels with this technique and some fatigue tests were carried out. Laser affected zones could be observed at the notch boundary but cracks were initiated from the notch tips and propagated steadily. The crack paths were very tortuous like natural small cracks. The experimental results showed that the femtosecond laser processing technique is useful to introduce a small notch and allows systematic investigation of microcrack behavior.

Measurement campaigns of the Max-Planck Advanced Study Group (ASG) in cooperation with the Center for Free Electron Laser Science (CFEL) at DESY-FLASH and SLAC-LCLS have established pnCCDs as universal photon imaging spectrometers in the energy range from 90 eV to 2 keV. In the CFEL-ASG multi purpose chamber (CAMP), pnCCD detector modules are an integral part of the design with the ability to detect photons at very small scattering angles. In order to fully exploit the spectroscopic and intensity imaging capability of pnCCDs, it is essentially important to translate the unprocessed raw data into units of photon counts for any given position on the detection area. We have studied the performance of pnCCDs in FEL experiments and laboratory test setups for the range of signal intensities from a few X-ray photons per signal frame to 100 or more photons with an energy of 2 keV per pixel. Based on these measurement results, we were able to characterize the response of pnCCDs over the experimentally relevant photon energy and intensity range. The obtained calibration results are directly relevant for the physics data analysis. The accumulated knowledge of the detector performance was implemented in guidelines for detector calibration methods which are suitable for the specific requirements in photon science experiments at Free Electron Lasers. We discuss the achievable accuracy of photon energy and photon count measurements before and after the application of calibration data. Charge spreading due to illumination of small spots with high photon rates is discussed with respect to the charge handling capacity of a pixel and the effect of the charge spreading process on the resulting signal patterns.

6 pages; International audience; In this study we develop a novel approach to tune nanopatterns on fused silica substrates exploiting the polarization dependence of the strongly localized near field of highly ordered triangular nanoparticle arrays. For this purpose such arrays were prepared by nanosphere lithography on fused silica substrates and subsequently irradiated with single 35 fs long laser pulses. The irradiation leads to the excitation of localized surface plasmon polariton resonances, followed by ablation of the nanoparticles and partially of the substrate. By this means, nanostructures were generated on the substrate surface, reflecting the local fields in the vicinity of the triangular nanoparticles. Depending on the applied fluence, small holes as well as extended nanostructures with dimensions well below the diffraction limit have been created. Furthermore, by rotating the linear polarization of the laser light by 90° with respect to the orientation of triangular nanoparticles, different plasmon modes have been excited, which in turn, alter the local field distribution. As a result, either nanochannels or bone like shaped nanogrooves in a chequered structure were generated on the fused silica substrates. Finite-difference time-domain simulations demonstrate, that the results can, in fact, be explained by the enhanced near field distribution, which is dominated by the excitation of localized surface plasmon polariton resonances in the triangular nanoparticles. It is shown, that the fluence and the polarization of the laser light are the key parameters in nanogroove and nanochannel formation.

Nanoscale laser processing of wide-bandgap materials with temporally shaped femtosecond laser pulses is investigated experimentally. Femtosecond pulse shaping in frequency domain is introduced and applied to two classes of shaped pulses relevant to laser nano structuring. The first class, characterized by a symmetric temporal pulse envelope but asymmetric instantaneous frequency allows us to examine the influence of the sweep of the photon energy. In contrast, asymmetric temporal pulse envelopes with a constant instantaneous frequency serve as a prototype for pulses with time-dependent energy flow but constant photon energy. In our experiment, we use a modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting in ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy (AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters. We find different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart. However, we do not observe pronounced differences between up- and down-chirped radiation in the measured structure diameters and thresholds.

Control of two basic ionization processes in dielectrics i.e. photo ionization and electron‐electron impact ionization on intrinsic time and intensity scales is investigated experimentally. In our experiment, we use a modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting in nanoscale ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy (AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters. We find different thresholds for surface material modification with respect to an asymmetric temporal pulse and its time reversed counterpart both showing a constant instantaneous frequency. However, we do not observe pronounced differences between up‐ and down‐chirped radiation (i.e. symmetric temporal pulse envelope but asymmetric instantaneous frequency) in the measured structure diameters and thresholds.

Control of two basic ionization processes in dielectrics i.e. photo ionization and electron–electron impact ionization on intrinsic time and intensity scales is investigated experimentally and theoretically. Temporally asymmetric femtosecond pulses of identical fluence, spectrum and pulse duration result in different final free electron densities. We found that an asymmetric pulse and its time reversed counterpart address two ionization processes in a different fashion. This results in the observation of different thresholds for surface material modification in sapphire and fused silica. We conclude that control of ionization processes with tailored femtosecond pulses is suitable for robust manipulation of breakdown and thus control of the initial steps of laser processing of high band gap materials.

Laser control of two basic ionization processes in dielectrics on intrinsic time and intensity scales with temporally asymmetric pulse trains is investigated. We create robust structures one order below the diffraction limit.

Transparent solids may absorb energy from a laser beam of sufficient high intensity. Several models are under consideration to describe the evolution of the free-electron density. Some of these models keep track of the energy distribution of the electrons. In this work we compare different models and give rules to estimate which one is applicable. We present the inclusion of a term in the multiple rate equation approach, recently introduced, describing fast recombination processes to exciton states. Moreover, we present experimental results with temporally asymmetric femtosecond laser pulses, impinging on a surface of fused silica. We found different thresholds for surface material modification with respect to an asymetric pulse and its time reversed counterpart. This difference is due to a different time-and-intensity dependence of the main ionization processes, which can be controlled with help of femtosecond shaped laser pulses.

© 2015 Author(s). Lensless x-ray microscopy requires the recovery of the phase of the radiation scattered from a specimen. Here, we demonstrate a de novo phase retrieval technique by encapsulating an object in a superfluid helium nanodroplet, which provides both a physical support and an approximate scattering phase for the iterative image reconstruction. The technique is robust, fast-converging, and yields the complex density of the immersed object. Images of xenon clusters embedded in superfluid helium droplets reveal transient configurations of quantum vortices in this fragile system.

Laser control of two basic ionization processes on fused silica, i.e., multiphoton ionization and avalanche ionization, with temporally asymmetric pulse envelopes is investigated. Control leads to different final electron densities/energies as the direct temporal intensity profile and the time inverted intensity profile address the two ionization processes in a different fashion. This results in observed different thresholds for material modification on the surface as well as in reproducible lateral structures being an order of magnitude below the diffraction limit (down and below 100 nm at a numerical aperture of 0.5). In this contribution, the morphology of the resulting structures is discussed.

We present experiments to prepare highly ordered nanochannels with subdiffraction dimensions on fused silica surfaces with femtosecond laser light. For this purpose, we exploit the strongly enhanced near field of highly ordered triangular gold nanoparticles. We demonstrate that after a single laser shot, 6 μm long nanochannels with a mean depth of 4 nm and an average width of 96 nm, i.e., well below the diffraction limit, are generated. These nanochannels are prepared by ablation, caused by the localization of the near field. The crucial parameters, besides the applied fluence, are the polarization direction of the incoming laser light with respect to the triangular nanoparticles and the size of the nanoparticles.

The Principles of femtosecond pulse tailoring for advanced material processing are reviewed. In our experiments we study laser control of two basic ionization processes in dielectrics on intrinsic time and intensity scales with temporally asymmetric pulse trains. We create robust structures one order below the diffraction limit.