Dependency of high-order harmonics to polarisation properties of many-cycle driving fields

University essay from Lunds universitet/Atomfysik; Lunds universitet/Fysiska institutionen

Abstract: Light-matter interactions can be used to probe both light and matter as they yield information on a system's state. On the ultrafast timescale, it is possible to employ these interactions to probe fast-occurring phenomena. In particular, one of the few tools for ultrafast spectroscopy are coherent attosecond XUV pulses: light pulses in the extreme ultraviolet range (XUV) whose typical duration is a few hundred attoseconds. These pulses are generated by a non-linear light-matter interaction process called high-order harmonic generation (HHG). It can even be possible to control the properties of generated harmonics by controlling those of the electric field that generated them. However, their dependency to properties of the electric field such as its polarisation state or its symmetries needs to be investigated to gain greater control over the properties of attosecond XUV pulses. This thesis examines the impact of driving HHG with elliptically polarised or two-colour laser fields. Simulations and experimental work focused on studying HHG spectra using Argon when varying laser parameters, including the driving laser field's intensity and polarisation state. Simulations used the time-dependent Schrödinger equation with a single Argon atom and assumed a single-active electron; experiments used a user-friendly source of harmonics based on a solid-state femtosecond infrared laser emitting many-cycle pulses. For the harmonic order closest in energy to Argon's first ionisation energy, simulations and experiments produced harmonic yields displaying a local maximum for certain driving fields ellipticities, with maximum yields found for slightly elliptically polarised fields instead of linearly polarised ones. Further simulations show that a linearly or circularly polarised two-colour field can generate sets of harmonics among all orders instead of only odd ones, or weakly generate orders multiple of 3 with elliptical polarisations, respectively. Harmonics are further enhanced or suppressed depending on the relative intensity and the phase delay between the field's colour components. These results add to existing knowledge on how to select, suppress, or enhance harmonics in Argon-based spectra by shaping the laser field driving HHG. Broader knowledge would ultimately allow appropriate harmonic selection for spectroscopic aims. Envisioned uses include following the temporal evolution of chiral systems using attosecond XUV pulses.

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