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Optical and quantum interferences in strong field ionization and optimal control

November 29, 2017 - 9:00am
David Foote
Institution of Physical Science & Technology, University of Maryland

For decades, ultrafast laser pulses have been used to image and control molecular dynamics in the strong field regime. These pulses are frequently employed in optimal control experiments, where the temporal profile of the laser pulse is iteratively shaped using an evolutionary algorithm. This algorithm eventually converges on the best pulse for controlling the dynamics -- the optimal control pulse. While this approach is experimentally powerful, a common problem with these experiments is that the optimal control pulse does not explain why it is optimal.

This thesis is concerned with "unpacking" a specific optimal control pulse to explain how it achieves its molecular control goals. The optimal control pulse studied in this work consisted of a pair of pulses, each of approximately 70 fs duration, separated by approximately 150 fs, and with a locked relative phase between the two peaks. Such a pair of pulses is referred to as a twin-peaked pulse (TPP). Ionization, the first step in this optimal control experiment, changes as the interpeak delay and relative phase of the TPP changes, due to two types of interference: optical interference (OI), in which the electric fields of the two peaks of the TPP interfere and change the pulse intensity, and quantum interference (QuI), in which the electron wavepackets produced by the two peaks interfere.

Two sets of experiments were developed to unpack this optimal control pulse. These experiments determined what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent dynamics of molecular ions. In the first set of experiments, the total ionization yield response to the interpeak delay and relative phase was measured using a time of flight mass spectrometer. It was found that OI was principally responsible for changing the ion yield, and that any contributions from QuI were obscured by experimental noise. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to cause surprisingly large variations in the OI; these changes occasionally mimicked QuI, highlighting the need for researchers in molecular control experiments to accurately characterize the temporal profile of their pulses.

The second set of experiments were motivated by a calculation of multiphoton ionization using time-dependent perturbation theory, which showed that the signatures of QuI in the ionic continuum vanish when measuring total electron yield, but appear when measuring energy-resolved electron yields. The second set of experiments probed ionization with a TPP, but detected the electron energy with a velocity map imager. The experiments were performed at high intensities (1013 to 1014 W/cm2) where the ac Stark effect tends to wash out the fine energy structures of QuI. The thesis ends by proposing a low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.

Professor Wendell Hill III, Chair/Advisor

Professor Howard Milchberg

Adjunct Professor Charles Clark, JQI

Professor Amy Mullin

Professor Ki-Yong Kim - Dean’s Representative

Atlantic 2115
College Park, MD 20742