UCLA

Optical-field ionization (OFI) is often used to produce plasmas for myriad laboratory applications. In this thesis, the electron velocity distribution functions of OFI helium plasmas are shown to be controlled by changing the wavelength and polarization of a sub 100 fs pump laser and ionization state of the plasma. Thomson scattering is used to measure the distinct electron velocity distributions of helium plasmas produced by linearly and circularly polarized laser pulses within a few inverse plasma periods after the plasma formation. In both cases, nonthermal and highly anisotropic initial electron velocity distributions (EVD) are observed that are consistent with expectations from OFI of both He electrons.Such OFI plasmas are used to study two important problems in plasma physics. First we show that the ability to initialize the EVD of electrons allows kinetic plasma instabilities to be studied in the laboratory. Second we carry out a comprehensive study of transfer of spin and orbital angular momentum in the process of second harmonic generation. We show that following the ionization but before collisions thermalize the electrons, the OFI plasma undergoes two-stream, filamentation, and Weibel instabilities that isotropize the electron distributions. The polarization-dependent frequency and growth rates of these kinetic instabilities, measured using Thomson scattering of a probe laser, agree well with the kinetic theory and simulations. Thus, we have demonstrated an easily deployable laboratory platform for studying kinetic instabilities in plasmas. The source and the angular momentum properties of the second harmonic beams generated from underdense OFI plasmas are studied. When laser beams with angular momentum interact with plasmas, one can observe the interplay between the spin and the orbital angular momentum. Here, by measuring the helical phase of the second harmonic 2ω radiation generated in an underdense plasma using a known spin and orbital angular momentum pump beam, we verify that the total angular momentum of photons is conserved in the generation of 2ω photons and observe the conversion of spin to orbital angular momentum. We further determine the source of the 2ω light by analyzing near field intensity distributions of the 2ω light. The 2ω images are consistent with these photons being generated near the largest intensity gradients of the pump beam in the plasma as predicted by the combined effect of spin and orbital angular momentum when Laguerre-Gaussian beams are used.