Cluster jets for quasi-phase matching in high-order harmonic generatio

Abstract

We propose and investigate a novel scheme for achieving quasi-phase matching (QPM) in high-order harmonic generation (HHG) via a density modulated cluster jet, as opposed to a modulated gas jet. A density modulation is expected to be easily achieved by placing an array of wires (grid) on the top of the nozzle. However, in our initial experiments, we did not observe any enhanced HH output using previously fabricated grids. These experiments suggest that, even if using a density modulated cluster jet remains promising for achieving QPM in HHG, an according demonstration requires a thorough investigation of several basic and essential aspects. In this thesis, we investigated three essential aspects. First, we presented a comprehensive modelling of cluster formation and systematically investigated all influences of various critical physical assumptions for the gas condensation in a supersonic nozzle using argon as an example. Using the proposed baseline model, we showed that the liquid mass fraction, g, is very insensitive with regards to the named variations in this model. The average cluster size, , was then retrieved from interferometric and Rayleigh scattering measurements by using the calculated g from the baseline model. Secondly, we performed a detailed experimental study on HHG in a supersonic argon jet. In order to identify and separate the contributions from clusters versus that of gas monomers in the jet to the generation of high-order harmonics, we characterized the harmonic spectra over a broad range of stagnation pressures at two different reservoir temperatures. From the measured spectra, we found that below an average cluster size, , of about 1000 atoms, HHG in clusters shows the same efficiency as in gas monomers. Only with larger clusters, HHG becomes less efficient. Lastly, we investigated QPM for HHG, specifically aiming at determining a proper quasi-phase matching modulation period for maximizing output pulse energy. We developed a one-dimensional, unified QPM model for HHG in a periodic-structured gaseous medium (argon). From the model, we showed that the optimum HH output pulse energy is obtained when transient QPM is provided in the leading edge of the drive laser pulse instead of at the peak intensity.