Charge Dynamics in Femtosecond Laser Filaments

The balance of self-focusing and ionization-induced de-focusing of a nonlinearly propagating laser pulse leads to a transversely confined, high intensity structure called a filament that can persist for long distances. The plasma channel left in the wake of the laser pulse has many interesting properties. One of these is the ability to generate short pulses of microwaves, which oscillate at gigahertz (GHz) frequencies. The thesis presents an experimental investigation of the microwave radiation. While terahertz radiation from plasmas generated by laser filamentation in air is well known, there are very few studies of the radiation in the microwave regime. The experiments measure several characteristics of the microwave emission in order to understand the source of the current formation in the plasma. These include the angular emission pattern of the radiation, its electric field polarization, the qualitative features of the time domain microwave waveforms, and their absolutely calibrated frequency spectrum up to a maximum frequency of 70 GHz. The dependence of the radiation on the electron-neutral collision rate is investigated by changing the air pressure in the filamentation region. The microwave measurements indicate that competition occurs between the nonlinear susceptibility of the air, and the conductivity of the plasma. As the pressure decreases, the microwave field increases by an order of magnitude. However it diminishes as the electron-neutral collision rate approaches the frequency of the microwave measurements. The dependence of the microwave radiation on the laser wavelength is studied by using two different laser systems to generate plasmas from pulses in the mid-infrared at 3900 nm, and the near-infrared at 800 nm, respectively. The microwave field strength due to the mid-infrared laser pulses is an order of magnitude larger than that due the near-infrared pulses. The relative increase may indicate that the microwave radiation is initiated by transfer of laser energy to the electrons via the ponderomotive force. Comparison of images of the plasma length to the microwave frequency spectrum shows that the generation mechanism involves changes in the current density over spatial scales of the plasma equal to its longitudinal extent. The mid-infrared laser is also used to demonstrate the dependence of the microwave radiation on the relative phase of the laser harmonics in a two color laser pulse. A third laser system uses a deformable mirror that is run by a genetic algorithm to shape the laser wavefront. The algorithm takes in the microwave amplitude as feedback and configures the wavefront to optimize the microwave emission from the plasma. The optimization leads to an increase in the length, width, and brightness of the plasma, which accompanies a doubling of the microwave field amplitude relative to the unoptimized case. Finally, the deformable mirror can also be used to arbitrarily configure the filaments in a multi-filamenting laser pulse simultaneously, enabling general studies of the mutual interactions of many co-propagating filaments. Together, the experiments imply that the energy transferred to the plasma by the ponderomotive force over subpicosecond timescales results in a long-lived, nanosecond plasma current with a phase velocity that approaches the group velocity of the laser. It evolves longitudinally along the length of the laser plasma, producing a forward-directed cone of radiation.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/151749/1/aengles_1.pd