M. Kremer

Wavelength dependence of sub-laser-cycle few-electron dynamics in strong-field multiple ionization

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N2 molecule by intense (0.3–0.5 PW cm-2), short (~35–40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

Evolution of dopant-induced helium nanoplasmas

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Minimizing dispersive distortions in carrier-envelope phase sweeping with glass wedges

A systematic experimental study is performed to examine the f-2f technique for sweeping the carrier-envelope phase (CEP) of few-cycle laser pulses by changing the amount of positive dispersion in the extracavity beam path. Slightly changing the dispersion not only changes the CEP but affects the entire spectral-phase function. As a result, large discrepancies are found between the true CEP as independently measured with a stereo-above-threshold-ionization spectrometer and the CEP detected by an f-2f interferometer when sweeping the phase with glass wedges. A new CEP-stabilization scheme is proposed and experimentally shown to significantly improve the performance of CEP sweeping.

Ignition of Doped Helium Nanodroplets in Intense Few-Cycle Laser Pulses

The ultra-fast dynamics of He nanodroplets (103–105 atoms) in intense (1…7 ×1014 W/cm2), few-cycle (∼10 fs), infrared (∼790 nm) laser pulses has been investigated as a function of the number of dopant rare-gas atoms, the laser intensity and the rare-gas species. We find the “ignition” behaviour predicted by theory for 20 fs resulting in the complete ionisation and disintegration of the droplet, otherwise entirely transparent, initiated by just a few, less than 5 dopant atoms.

Pump-Control Experiments to Enhance the Electron Localizability in Dissociating H 2 with Phase-Stable Laser Pulses

The first two-pulse measurements with carrier-envelope-phase (CEP) stabilized laser pulses on H2 were recorded with a reaction microscope. The role of a coherent wave packet in the dissociating H2+ for the charge localization is investigated.

Electron localization in fragmentation of H2 with CEP stabilized laser pulses

Fully differential data on ionization and dissociation of H2 in ultra-short (~ 6 fs), linearly polarized, intense (~ 4.1014W/cm2) laser pulses with stabilized carrier-envelope-phase (CEP) have been measured using a reaction microscope. Depending on the CEP of the laser pulses we see a clear asymmetry in the emission direction of the created protons. Contrary to earlier measurements by Kling et al. [1] we observe the highest asymmetry for kinetic energy releases (proton energy) between 0-2 eV. This excludes the electron re-collision mechanism suggested in [1] as dominant excitation channel and requires another explanation.