Quantum electrodynamics study of polarization effects on the properties of high harmonics of an intense ultrashort laser pulse interacting with relativistic electrons

In the framework of strong-field quantum electrodynamics, the angular and spectral distribution of the polarized photons scattered from electrons interacting with few-cycle laser pulse are investigated using density-matrix theory. Effects are taken into consideration about various factors, including the ellipticity of ultrashort laser pulse, carrier envelope phase (CEP), electron wave packet, and incident electron energy. In the case of a linearly polarized laser pulse interacting with electrons of not-so-high energy, the angularly resolved spectra of the scattered photons within the polarization plane are entirely contributed by the E-polarized photons, which is destroyed when GeV electrons are used. Contributions from the B-polarized photons are weaker and located at odd harmonics in the magnetic-field plane. For a circularly polarized interacting pulse, there is a certain polar angle range within which the angularly resolved spectra near the backward-scattering direction is entirely contributed by photons having helicity opposite to that of the laser pulse. Out of this range, the angular spectrum is relatively weaker and mainly comes from polarized photons having the same helicity to the laser pulse. The broadening of the electron momentum in the transverse direction causes a extension of the radiation angle, while in the longitudinal direction, electron's momentum broadening blurs the interference structure. For an elliptically polarized pulse with CEP ≠0, the maximum of frequency-integrated angular distribution in the transverse plane is no longer along the direction of the peak of the vector potential.