The theory of nonlinear Thomson scattering is presented. A model for the scattered light is developed. The orthogonal polarizations of the second harmonic of the scattered light are examined. The predictions of the model are compared to measurements by our group in collaboration with the Extreme Light Laboratory at the University of Nebraska-Lincoln (UNL). The veracity of the theory and model are confirmed by comparison to the experimental data.

Thomson scattering from free electrons in a high-intensity laser focus has been widely studied analytically, but not many measurements of this scattering have been made. We measure polarization-resolved nonlinear Thomson scattering from electrons in a high-intensity laser focus using a parabolic mirror. The weak scattering signal is distinguished from background noise using single-photon detectors and nanosecond time-resolution to distinguish a prompt scattering signal from noise photons. The azimuthal and longitudinal components of the fundamental, second, and third harmonics were measured. Our measurements reasonably match theoretical results, but also show some asymmetry in the emission patterns.

We investigate nonlinear Thomson scattering generated by intense laser pulses focused in noble gases. Electrons are ionized from low-density helium or argon early during the laser pulse and scatter light out the side of the focus. We measure fundamental, second, and third harmonic light focus over nearly the entire emission sphere. We investigate the influence of electron bunching, when individual atoms ionize multiple electrons sequentially as the laser field increases early during the laser pulse. Simulations suggest that correlation between electron positions for the ten electrons ionized from argon should distort the spatial pattern of nonlinear Thomson scattering relative to the two-electron pattern observed for helium. Preliminary experimental measurements do not reveal a difference in emission patterns between helium and argon. However, our experimental efforts were hampered by prepulses in our laser system, which could cause electrons to ionize ahead of the main pulse and disrupt the coherence between electrons. In the future, we hope to improve the pulse temporal contrast of the laser system and revisit this experiment. We also explore observed distortions in the angular emission patterns of nonlinear Thomson scattering. These persistent distortions arise presumably from subtle defects in the laser field, which can imprint on the angular distribution of the scattered light. Possibilities include variations in laser focal shape, spatial chirp, and laser-field polarization. We set out on a systematic experimental course to identify laser beam characteristics that may contribute to such distortions in the angular emission patterns. We develop experimental tests which have implicated our wave plate, used to rotate linear laser polarization, as a likely source of distortion. Further tests on the wave plate are required to confirm and explain how it affects the angular distribution of nonlinear Thomson scattering.