Nonlinear optics of plasmas in the relativistic regime.

Abstract

With the advent of high-intensity short-pulse laser technology, focused laser intensity exceeding 10<super>18</super> W/cm<super>2</super> has been achieved. Under such a high laser intensity, electrons quiver at velocities approaching the speed of light in vacuum and, thus, relativistic increase of electron mass and the magnetic field of the laser can affect the electron dynamics significantly. The relativistic motion of electrons has three main effects on laser-plasma interaction. First, because the electron quiver motion in the laser field becomes highly nonlinear, harmonics of the laser pulses can be generated through nonlinear Thomson scattering in a plasma. Second, due to the dependence of the refractive index on electron mass, the spatially- and temporally-dependent modification of the refractive index for a laser pulse propagating in a plasma results in relativistic self-focusing and relativistic self-phase modulation of the laser pulse. Third, the laser ponderomotive force of a tightly-focused high-intensity short laser pulse can drive a plasma wave longitudinally and create a plasma density depression transversely. The combination of the last two effects also leads to Raman forward scattering instability and envelope self-modulation. In this thesis, all of these phenomena were observed and characterized experimentally. Harmonics generated by nonlinear Thomson scattering were identified. Relativistic-ponderomotive self-channeling of a laser pulse was observed. The formation of a plasma waveguide following this process was diagnosed and the guiding of an intense laser pulse in such a waveguide was demonstrated. In addition, electron plasma waves excited through Raman forward scattering instability were characterized and various damping mechanisms were investigated. Lastly, the acceleration of electrons in a self-modulated laser wakefield was studied. The dynamics of electron acceleration is understood by comparing the characteristics of the generated electron beam and the results of test particle simulations. These results are crucial to applications involving high-intensity laser pulses in a plasma, such as laser fusion, high brightness coherent x-ray sources, and compact laser-plasma-based particle accelerators.