Attosecond and zeptosecond x-ray pulses via nonlinear Thomson backscattering

Nonlinear Thomson backscattering of an intense circularly polarized laser by a counterpropagating energetic electron is investigated. The results show that in the scattering of a non-tightly-focused laser pulse with an intensity around 10¹⁹ W∕cm² and a pulse duration of 100 fs full width at half maximum by a counterpropagating electron with an initial energy of 10 MeV, a crescent-shaped pulse with a pulse duration of 469 as and the photon energy ranging from 230 eV to 2.5 keV is generated in the backward direction. It is shown that the radiated pulse shape and monochromaticity can be modified by changing the laser beam waist, while in the case of a tightly focused laser field, a single peak pulse with a shorter duration and better monochromaticity can be obtained. With increase of the electron initial energy, the peak power of the radiated pulse increases and the pulse duration decreases. An isolated powerful zeptosecond (10⁻²¹ s) pulse with a peak power of about 10¹⁰ W∕rad² and photon energy up to several MeV can be obtained with a 250 MeV electron.