Optimization of High-order Harmonic Generation for Attosecond Science

Abstract: High-order harmonic generation is a highly nonlinear, though inherently inefficient, process which can lead to emission of coherent, broadband extreme ultraviolet radiation in the form of attosecond pulses. Attosecond pulses are crucial for experiments investigating photoionization dynamics on the femto- and attosecond timescales. As attosecond research tends towards increasingly complex light-matter interactions, demands on high flux attosecond sources grow. This thesis deals with light-matter interactions in the non-perturbative and perturbative regimes. Optimal generation of high-order harmonics in gases is studied, and the attosecond pulses are applied in two-photon pump-probe photoelectron interferometry schemes to unravel photoionization dynamics on the intrinsic timescales of the electron.The first part of this thesis focuses on optimization of the conversion efficiency in high-order harmonic generation in gases, with emphasis on macroscopic phase-matching effects. We explain the large variety of gas target designs in the literature through an analytic model. The model predicts, independently of the driving laser focusing geometry, that efficient high-order harmonic generation is possible for a wide range of densities and medium lengths, if these follow a hyperbolic relation. The model suggests the existence of two phase-matching regimes with similar efficiency but different spatial and temporal characteristics of the emitted extreme ultraviolet radiation. We verify the model for a wide range of generation parameters experimentally and using numerical simulations.The second part of this thesis concerns the application of attosecond pulse trains, consisting of high-order harmonics, to infer information about electron correlations in atoms. Photoionization dynamics occurring on the femto- and attosecond timescales are probed by measuring the amplitude and phase of oscillations in the photoelectron signal, induced by path interference of two-photon transitions. Two interference techniques are used: First, Reconstruction of Attosecond Beatings By Interference of Two-photon transitions (RABBIT) is used to study (i) photonionization time delays across the 4d giant dipole resonance in xenon, (ii) resonant below-threshold two-photon ionization of the 1s3p, 1s4p and 1s5p Rydberg states in helium and (iii) autoionization dynamics from the 3s13p64p Fano resonance in argon. Secondly, to fully characterize mixed photoelectron quantum states, a quantum state tomography protocol for photoelectrons (KRAKEN) is developed theoretically and tested experimentallyfor non-resonant photoionization of helium and argon.