Enhancement effect of frequency chirp on vacuum electron-positron pair production in strong field

Abstract

In this review article, we show an important aspect of electron-positron pair production from vacuum under strong background field where the frequency chirping plays a key role in enhancing the pair production. A series of researches on the enhancement effect of frequency chirp on electron-positron pair production in strong field is summarized. Three approaches are introduced, i.e. the Dirac-Heisenberg-Wigner formalism used to treat the spatial inhomogeneous field or/and multidimensional homogeneous time-dependent field, quantum Vlasov equation to cope with the one-dimensional homogeneous time-dependent field, and the computation quantum field theory employed to study the problem with external potential. Some interesting results about the momentum spectrum structure of created particle and the yielding of pair numbers are demonstrated for various different field parameters such as field strength and central frequency, in particular their significant influence on results when the frequency chirping form or/and strength are changed. In general, the number density can be improved by 2-3 orders of magnitude with the strengthening of frequency chirping in comparison with that without chirping for low frequency field, which is attributed to the effect that the dynamically assisted mechanism plays a significant role since the chirping expands the frequency spectrum of field. For high frequency field, however, this effect is suppressed so that the number density is enhanced by about a few times. For spatially inhomogeneous field, field changing on a small scale does not make the number density so high and the frequency chirping can enhance the yield in the order of magnitude, while the field changing on a large scale makes the number density to approach to that of homogeneous field and the chirping increases the yield by a few times. These numerical results can be understood by the Wentzel-Kramer-Brillouin (WKB) approximation and the structure of turning points. Finally the possible applicable prospects of the electron-positron pair production by the frequency chirping are presented briefly.