Light-Matter Interaction on the Attosecond Timescale
Sammanfattning: Attosecond science refers to physical processes that occur on the natural timescale of electron motion in atomic and molecular systems. Attosecond time resolution can be obtained experimentally through a process called high-order harmonic generation where sharp attosecond pulses are formed in the time domain. Attosecond pulse trains contain many attosecond pulses with a constant pulse-to-pulse separation. The attosecond pulse trains provide a unique combination of temporal and spectral properties, since they correspond to coherent odd harmonics in the spectral domain. It is possible to use these temporal properties to ionize an atom at specific times, but it is also possible to use the spectral properties and tune a harmonic at a specific atomic resonance. In either case, the emitted photoelectrons can be probed with a coherent infrared field, and attosecond temporal information can be obtained. The work presented in this thesis shows that attosecond pulse trains can be used to study the phase variation of various two-photon ionization processes and, thus, the temporal properties of emitted photoelectrons. The delay in photoemission, the so-called Wigner delay, is discussed from a theoretical stand-point, and it is explained how it relates to the experimental method. The generation of attosecond pulses can be controlled using two-color laser fields for the high-order harmonic process. Experimental and theoretical results are presented, where the two-color field consists of a fundamental laser field, with an intensity of ~10^14 W/cm^2, and a second harmonic field with a relative intensity of ~10%. The delay between the two fields can be used to smoothly alter the spectral content, the divergence and the temporal properties of the attosecond pulses. Alternatively, when the second harmonic field is weak, ~0.1% relative intensity, it can be used to probe the one-color high-order harmonic generation process by considering the phase offset of the weak even-order harmonics. The established RABITT method is compared experimentally to the two-color probing technique and inconsistencies are reported close to the cutoff. Theoretical work presented in this thesis, shows that the inconsistencies can be explained using a quantum mechanical model for the two-color high-order harmonic generation. Finally, the transition from many attosecond pulses to few attosecond pulses using a second harmonic field in combination with a few-cycle fundamental laser field is reported.
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