Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

In the past year, we have been able to generate with our Femtolaser V system (800 nm, 5 mJ, 1 kHz, CEP stabilized) high harmonic spectra up to 130 eV pulses in a filament created in a differential pumped gas cell. We plan to extend this region with our TOPAS system. We have already measured CEP (carrier envelope phase) stabilized laser pulses of 2400 nm up to 1mJ. The setup will allow us to extend the HHG cut-off region up to 400 eV.

The scientific goals are divided into 3 different axes.

In a first project we want to study excitation processes and inner-shell ionization as well as inner shell excitation processes of molecules and atoms.

Therefore we will use (femtosecond) pump - (sub-femtosecond) probe experiments and measure the electron spectrum of both laser pulses together. We will obtain in the low energy part of the electron spectrum the ATI (above threshold ionization) spectrum of the femtosecond pump laser, the high energy part will measure the probe pulse. We are only limited to laser intensities where the energy distribution of electrons from the ATI-spectrum does not overlap the electron energy spectrum from the probe pulse. This restriction can be overcome when the momentum distribution from the electrons of the pump and probe pulse is different. We want to upgrade our VMI (velocity map imaging) spectrometer to measure both contributions. With this technique we will be able to study inner-shell ionization, inner shell excitation processes of molecules and atoms. This offers us complementary information to HHG experiments.

In a second project we want to visualize processes taking place in a sub cycle of a laser.

We have started atom ionization/excitation experiments with polarization gated laser pulses and have been the first to publish VMI electron spectra. These laser pulses are composed of circularly polarized sections at the leading and trailing edges of the pulse and of an experimentally defined linearly polarized central part. The central part can be restricted down to a fraction of an optical cycle. Processes taking place only at linear polarization can be restricted in time down to a half cycle. We want to use this method for different wavelengths as well as ionization processes with laser pulses with sub-cycle linear polarization.

We want to better understand HHG generation in a filament and continue the experiments we have started recently with 800 nm. Later we will extend these experiments to other wavelengths generated by the TOPAS system. There we want to study the effect of atomic/molecular resonances as well as the possibility to amplify a HHG signal in a filament. Measurements, where the intensity as well as the wavelength dependent HHG spectra are taken at the same time will be an ideal tool to visualize atom/molecular high laser field physics.