Two-photon double ionization of neon using an intense attosecond pulse train

We present the first demonstration of two-photon double ionization of neon using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a photon energy regime where both direct and sequential mechanisms are allowed. For an APT generated through high-order harmonic generation (HHG) in argon we achieve a total pulse energy close to 1 $\mu$J, a central energy of 35 eV and a total bandwidth of $\sim30$ eV. The APT is focused by broadband optics in a neon gas target to an intensity of $3\cdot10^{12}$W$\cdot$cm$^{-2}$. By tuning the photon energy across the threshold for the sequential process the double ionization signal can be turned on and off, indicating that the two-photon double ionization predominantly occurs through a sequential process. The demonstrated performance opens up possibilities for future XUV-XUV pump-probe experiments with attosecond temporal resolution in a photon energy range where it is possible to unravel the dynamics behind direct vs. sequential double ionization and the associated electron correlation effects

Synthesis of graphene and graphene nanostructures by ion implantation and pulsed laser annealing

In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si− plus C− and Au + ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even laser frequency. Optimum II and PLA conditions are identified and possible mechanisms for selective synthesis of G, FLG, and carbon nanostructures are discussed

Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

Citation: Savelyev, E., Boll, R., Bomme, C., Schirmel, N., Redlin, H., Erk, B., . . . Rolles, D. (2017). Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser. New Journal of Physics, 19, 13. doi:10.1088/1367-2630/aa652dIn pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. Wediscuss in detail the necessary data analysis steps and describe the origin of the timedependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment

Effects of buffer layers on the structural and electronic properties of InSbInSb films

We have investigated the effects of various buffer layers on the structural and electronic properties of nn-doped InSbInSb films. We find a significant decrease in room-temperature electron mobility of InSbInSb films grown on low-misfit GaSbGaSb buffers, and a significant increase in room-temperature electron mobility of InSbInSb films grown on high-misfit InAlSbInAlSb or step-graded GaSb+InAlSbGaSb+InAlSb buffers, in comparison with those grown directly on GaAsGaAs. Plan-view transmission electron microscopy (TEM) indicates a significant increase in threading dislocation density for InSbInSb films grown on the low-misfit buffers, and a significant decrease in threading dislocation density for InSbInSb films grown on high-misfit or step-graded buffers, in comparison with those grown directly on GaAsGaAs. Cross-sectional TEM reveals the role of the film/buffer interfaces in the nucleation (filtering) of threading dislocations for the low-misfit (high-misfit and step-graded) buffers. A quantitative analysis of electron mobility and carrier-concentration dependence on threading dislocation density suggests that electron scattering from the lattice dilation associated with threading dislocations has a stronger effect on electron mobility than electron scattering from the depletion potential surrounding the dislocations. Furthermore, while lattice dilation is the predominant mobility-limiting factor in these nn-doped InSbInSb films, ionized impurity scattering associated with dopants also plays a role in limiting the electron mobility.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87622/2/043713_1.pd

Generation and spatiotemporal characterization of ultrashort vortex pulses

Some light beams rotate as they propagate. If it is not the polarization vector, but the phase structure that rotates, the beam is said to carry orbital angular momentum (OAM). Such beams exhibit a helical phase front, where the phase rotates around a symmetry center. Because the phase in the center is undefined (and the intensity there is therefore zero), it is often termed a phase singularity or optical vortex by analogy to superfluidic vortices. Vortex beams [1,2] and more specifically ultrashort (few cycle) vortex pulses [3] have recently attracted strong interest