2 Ultrafast and Nonlinear Phenomena:
   Atoms, Molecules and Plasma
Group coordinator:

This research focus is devoted to ultrafast and nonlinear processes in atoms, molecules, clusters and plasmas induced by short laser pulses. It covers a broad range of intensities from ultra-strong fields where the external field is the dominant interaction down to light-matter interaction at perturbative field strengths.

Few-particle interactions are typically investigated with novel spectroscopic methods yielding comprehensive kinematical information, and allowing for detailed comparison with elaborate theories. The detailed understanding of few-particle and plasma dynamics in ultrafast and strong laser pulses has proven to be one of the keys to novel applications in nonlinear optics.

Fundamental processes in strong fields presently focus on multi-electron dynamics and correlations in atoms, molecules and clusters, including multiple ionisation, harmonic generation and strong field reaction control. Processes in ultra-strong fields focus on relativistic effects in multi-electron ionisation of atoms and ions, and on relativistic plasma dynamics including the generation and application of laser accelerated protons (proton imaging) and coherent EUV radiation (“X-ray lasers (XRL)”)

Studies of ultrafast processes and structural changes are concerned with the investigation of nuclear and electronic dynamics, as well as ultrafast chemical reactions. The work focuses on molecular clusters and biologically relevant molecules in the gas phase, molecular liquids and polyatomic molecules in the condensed phase using novel methods of multidimensional spectroscopy.

Present activities are grouped in four projects, focusing on the dynamics of fully ionised matter (plasmas), on the transitions between neutral and ionised matter, and on the light-induced dynamics of molecular motion and fragmentation.

Fully ionised matter

Investigations involve both the internal dynamics of plasmas created by relativistic laser intensities, and the creation and control of secondary radiation, specifically protons and EUV-radiation. After implementation of the unique possibility to synchronize two separate high-power lasers the medium term research strategy involves shaping the energy spectrum of accelerated protons while simultaneously searching for methods to reduce the energy required for a repetitive production of MeV ion beams. Optimisation of proton emittance and energy will be studied through variation of laser pulse parameters, using micro-structured, multi-layered or spherical targets. Once the proton imaging technique is established it will be used to study field structures in relativistic plasmas, such as the predicted mechanisms of electron flow, soliton structures, laser filamentation and strong magnetic fields. Main activities in 2.1, close collaboration with 2.2, 1.2 and 4.2. The sub-project “table-top X-ray laser” focuses on the optimisation of coherent EUV-emission from laser-created plasmas. The next steps towards the long-term vision (stable saturated lasing around 13 nm with microjoule pulse energies, high beam brilliance and high repetition rate between 10 Hz and 1 kHz) involve the investigation of seeded amplification, using seed pulses generated by high harmonics (HHG), in a saturated 10 Hz operation pumped by shaped pulses from the MBI Ti:sapphire laser. After that, the step towards higher repetition rates will depend on research towards novel pump lasers, providing shaped ps pulses with energies on the order of Joule and repetition rates up to 1 kHz. Applications for interferometric studies (including plasma diagnostics) are foreseen as well as in diagnostics and component characterization in the industrially relevant spectral region around 13 nm. Main activities in 2.1, close collaboration with 3.3.

Strong field ionisation dynamics

The general goal is to understand the interplay between electron-electron correlation and dynamics in ultra-strong fields. The prevailing experimental method is detailed fragment spectroscopy, co-pioneered by MBI research groups, which will in the mid-term future be applied to new targets and in new intensity regimes in order to explore the limits of existing theoretical models. Specifically, this includes the study of the influence of molecular structure on the multi-electron dynamics, the yet unexplored transition regime between multi-photon and single photon double ionisation, the electron dynamics on the single- and sub-single-optical cycle time scale as well as multiple ionisation of atoms and ions at relativistic light intensities. Tools to realise these objectives will be few-cycle pulses with stabilized and adjustable carrier-envelope phase, ultra-short high-order harmonic light pulses, relativistic laser intensities, and ionic trapped targets. Medium term applications will include the absolute in-situ determination of ultra-high laser intensities through relativistic field ionisation of atoms and ions (“atomic intensity probe”), trying to solve a long-lasting problem in ultra-high intensity laser research. The experimental work will be accompanied and augmented by in-house theory. Main activity in 2.2, close collaboration with 1.2 and 4.1.