Phase-modulated High Harmonic Generation

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Extreme ultraviolet (XUV) light sources based on high harmonic generation (HHG) from high-power femtosecond laser pulses have become profitable tools for new directions in atomic and molecular physics, and attosecond science. Although from HHG corresponding XUV pulses are readily available for experiments, pulse manipulation such as split-and-delay units for pump-probe experiments, or specific phase-control of pulse pairs remain challenging in the XUV. This is one reason why sophisticated spectroscopic techniques such as coherent (multidimensional) spectroscopy have not been performed so far at XUV wavelengths. Moreover, experiments on gas-phase targets which can exploit rich options in electron and ion detection, are challenging because of the low XUV photon flux of HHG sources. In this project we will break new grounds in generating XUV pulse pairs for coherent XUV spectroscopy based on a phase modulation technique and higher harmonic lock-in detection. In this approach only the infrared driving laser field is manipulated avoiding any phase and delay control acting on the XUV pulses. The project aims at demonstrating its technical feasibility, elucidating its limitations and performing first experiments characterizing XUV phase control properties. Experiments are proposed to proof the applicability in different directions and using complementary detection techniques. On the one hand, XUV wave packet interferometry on atomic and molecular beams as well as gas targets will be performed revealing coherent dynamics of excited states. We will use both photoelectron spectroscopy as well as transient absorption as the most commonly used detection techniques in XUV time-resolved experiments. On the other hand, the phase modulation technique will be used as a new approach improving sensitivity and selectivity in experiments for the reconstruction of attosecond beating by two-photon transitions (RABBITT), opening prospective experiments on higher order sideband beat structures