Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~10(8) to 10(11) atoms. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.

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.

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.

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 twodimensional 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.

No abstract

No abstract

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.

Bichromatic polarization-shaped femtosecond laser pulses are used to control three-dimensional photoelectron momentum distributions (3D-EDs) from resonance enhanced multi-photon ionization of potassium atoms. The light fields consisting of two spectral bands with different ellipticity are produced using an ultrafast polarization pulse shaper equipped with a custom polarizer in the Fourier plane. The tomographically reconstructed 3D-EDs from ionization with counterrotating circularly or orthogonal linearly polarized bichromatic laser pulses show different angular momentum superposition states at four distinct photoelectron energies. The analysis of the measured 3D-EDs reveals that the underlying physical mechanism is based on the interplay of ionization pathway selection via quantum mechanical selection rules for optical transitions and intrapulse frequency mixing of the spectral bands with different ellipticity. multiple ionization pathways by energetic separation of different angular momentum target states. Control of the ionization pathway along with energy-and angle-resolved detection allows us, on the one hand, to generate individual angular momentum free electron wave packets using CNR-CP bichromatic pulses and, on the other hand, to create unusual angular momentum superposition states using O-LP bichromatic pulses. In the experiment, bichromatic polarization-tailored fields are produced using a f 4 polarization pulse shaper [35] equipped with a custom polarizer in the Fourier plane [36,37]. Multiple photoelectron momentum images are recorded employing a velocity map imaging (VMI) spectrometer [38] and subsequently combined to reconstruct the 3D-EDs using a tomography algorithm [39][40][41]. Positive and negative time delays between the two colors are introduced to discriminate resonant from non-resonant ionization and to observe signatures from spin-orbit wave packets (SOWPs) in the 3D-EDs. A similar setup was recently used to produce electron vortices with a sequence of two one-color CNR-CP femtosecond laser pulses [42]. In that experiment, the minimal time delay in the sequence was given by the pulse length in order to avoid the creation of linear polarization in the overlapping interval. However, CNR-CP bichromatic fields, consisting of temporally overlapping left-(L-CP) and right-handed circularly (R-CP) polarized disjoined spectral bands do not create linearly polarized light. This feature allows us to observe photoelectrons from REMPI by overlapping bichromatic CNR-CP pulses such that frequency mixing between photons of opposite helicity can occur. For example, we demonstrate that 3D-EDs from three-photon ionization with two red L-CP photons and one blue R-CP photon are observed in a specific kinetic energy window.We start in section 2 with a theoretical discussion of the 3D-EDs from 1+2 REMPI with bichromatic polarization-shaped laser pulses. Section 3 introduces the experimental strategy based on the combination of polarization pulse shaping and tomographic photoelectron imaging. The experimental ...