The relativistic dynamics of an electron subjected to the classical electromagnetic field of an ultrashort laser pulse is studied theoretically at arbitrary intensities. Frequency modulation effects associated with the nonlinear relativistic Doppler shift induced on the backscattered radiation are analyzed in detail. For circular polarization, an exact analytical expression for the full nonlinear spectrum is derived. For linear polarization, it is found that the scattering of coherent light by a single electron describing a well-behaved trajectory can yield anharmonic spectra when the laser ponderomotive force strongly modulates the electron's proper time. At ultrahigh intensities, these nonlinear relativistic spectra exhibit complex structures. In addition, the temporal laser pulse shapes best suited to generate narrow Compton backscattered spectral lines at ultrahigh intensities are discussed. ͓S1063-651X͑96͒02009-0͔
The validity of the concept of laser-driven vacuum acceleration has been questioned, based on an extrapolation of the well-known Lawson-Woodward theorem, which stipulates that plane electromagnetic waves cannot accelerate charged particles in vacuum. To formally demonstrate that electrons can indeed be accelerated in vacuum by focusing or diffracting electromagnetic waves, the interaction between a point charge and coherent dipole radiation is studied in detail. The corresponding four-potential exactly satisfies both Maxwell's equations and the Lorentz gauge condition everywhere, and is analytically tractable. It is found that in the far-field region, where the field distribution closely approximates that of a plane wave, we recover the Lawson-Woodward result, while net acceleration is obtained in the near-field region. The scaling of the energy gain with wave-front curvature and wave amplitude is studied systematically.
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