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Plasmonic effects in nanosized particles enhance the interaction between light and matter due to the localized surface plasmon resonance, with potential applications such as all-optical transistors and optical computers. Commonly, the dynamics of nanoparticles’ optical properties are assessed via pump-probe spectroscopy, where a plasmonic structure is exited by an initial laser, the pump. Thereafter a second, less intense laser, a probe, interacts with the now excited structure at a time delay. Through measurements of the probelight transmitted by the matter, the optical dynamics of the structure are monitored. Similar methodologies can potentially be used for other applications as well, such as all-optical switching. This study focuses on an implementation of a numerical finite element method model simulating a pump-probe experiment to predict the effects of different geometries and evaluate experimental data. The simulations are split into three parts. Initially, periodically spaced nanoparticles are excited by the pump laser. Then the model estimates the internal thermal dynamics of the excited nanoparticles and in turn, determines the change in complex permittivity. Lastly, the probe-matter interaction is modeled. To evaluate the model, a comparison with another model was performed. Furthermore, simulations of periodically spaced gold dimer nanoparticles in air were done to investigate how dimers affect transmitted light. For a probe light polarization rotated 45◦ against the axis parallel to the dimer, a change in rotation of 6◦ over 35 fs was induced by the pump, indicating a potential switching mechanism.

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Laser high-order harmonic generation in the presence of relatively weak interfering light is investigated. The interfering pulses intersect the primary harmonic-generating laser pulse at the laser focus. The interfering light creates a standing intensity and phase modulation on the field, which disrupts microscopic phase matching and shuts down local high harmonic production. Suppression of the 23rd harmonic (by two orders of magnitude) is observed when a counter-propagating interfering pulse of light is introduced. A sequence of counter-propagating pulses can be used to shut down harmonic production in out-of-phase zones of the generating volume to achieve quasi phase matching. Harmonic emission is enhanced in this case. A new high-power laser system with higher pulse energy has been constructed to further investigate quasi phase matching of high-order harmonics generated in difficult-to-ionize atomic gases (e.g., neon as opposed to argon). The new system can also be used to study harmonic generation in ions. A new counter-propagating beam produces a train of 5 pulses with regulated timing. In preliminary tests, the new system has produced high harmonics up to the 65th order in neon. This should increase with additional adjustments to the laser system. The high-order harmonics have also demonstrated to be useful for polarized reflectometry measurements of optical surfaces in the extreme ultraviolet (EUV) wavelength range.

The time-resolved electrical conductivity of a short-pulse generated plasma filament in air was studied. Close-coupled metal electrodes were used to discharge the stored energy of a high-voltage capacitor and the resulting microsecond-scale electrical discharge was measured using fast current sensors. Significant differences in the time dependence of the current were seen with the two electrode geometries used. Using sharp-tipped electrodes additional peaks in the time-resolved conductivity were seen, relative to the single peak seen with spherical electrodes. We attribute these additional features to secondary electron collisional ionization brought about by field enhancement at the tips. Additional discrepancies in the currents measured leaving the high-voltage electrode and that returning to ground were also observed. Implications for potential laser-induced discharge applications will be discussed.