Identifier

etd-06232017-140117

Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

We present a theoretical study of the influence of resonant enhancement on high harmonic generation for harmonics near the ionization threshold of helium and argon atoms. This is done by solving the time-dependent Schrodinger equation (TDSE) for the interaction between an atom and a driving laser pulse. By varying parameters of the driving laser pulse, we are able to identify the enhancement of harmonics resulting from multiphoton resonances between the ground state and Stark-shifted excited states of the atom. In this way, spectral and temporal signatures of resonant enhancement are identified and highlighted. We also study trajectory dynamics in HHG. For resonances occurring at the peak intensity of the driving laser pulse, we use time-frequency analysis on the emission to show that both long and short quantum path contributions to the harmonic yield can be enhanced. In this case, we also show that the phase of the long quantum path can be perturbed by the resonant interaction. For resonances occurring at intensities below the peak intensity, we demonstrate a new approach to studying resonant enhancement in both the time and spectral domain. This approach allows us to separate the long and short quantum path spectrally, allowing for unique insight into quantum path dynamics during the interaction with the strong laser field. We compare our results to those of recent experiments and find good agreement. We also present a theoretical study of the interaction of extreme ultraviolet (XUV) pulses with a resonant macroscopic medium in transient absorption. In order to do this, the coupled TDSE and Maxwell's wave equation (MWE) are solved for an electric field propagating through a gas of atoms. We first show that a resonant medium can act to greatly alter the absorption line shape of an XUV pulse, when the medium is optically thick and the resonance is long-lived. This is demonstrated to be an effect of resonant pulse propagation, a temporal reshaping of light pulses interacting with long-lived resonances (relative to the duration of the pulse). The evolution of this spectral and temporal reshaping with increased propagation distance and gas pressure is investigated, both theoretically and experimentally. We next demonstrate a new optical tool that we have developed for controlling the spatial properties of XUV light. This optical tool uses an infrared pulse to tailor the emission direction of free-induction decay in a gas after a resonant XUV pulse excites the atoms. This technique is explored in both theory and experimental work, demonstrating the spatial control in two different geometries. It is also shown that this technique can be used to spectrally study the atomic response of atoms independently from the excitation pulse. This is a unique scenario, as typically the atomic response is measured as absorption in the spectrum of the excitation pulse.

Date

2017

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Gaarde, Mette

DOI

10.31390/gradschool_dissertations.4478

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