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In this review article, we show an important aspect on pair production from vacuum under strong background field where the frequency chirping plays a key role to enhance the pair production. A series researches on the enhancement effect of frequency chirp on electron-positron pair production in strong field are exhibited and summarized. Three approaches are introduced, Dirac-Heisenberg-Wigner formalism is used to treat the spatial inhomogeneous field or/and multidimensional homogeneous time-dependent field, quantum Vlasov equation solving is used to cope with the one-dimensional homogeneous time-dependent field, and the computation quantum field theory is employed to study the problem with external potential, respectively. Some interesting results for the momentum spectra structure of created particles and the yielding of pair numbers are demonstrated for various different field parameters such as field strength and central frequency, in particular its significant impact on results when the frequency chirping form or/and strength is changed. In general the number density can be improved 2-3 orders with frequency chirping compare to that without chirping for low frequency field, which is attributed to the effect that the dynamical assisted mechanism plays a significant role since the chirping expand the frequency spectrum of fields. For high frequency field, however, this effect is suppressed so that the enhancing of number density is about a few times. For spatial inhomogeneous fields, small scale makes the number density no high and the frequency chirping enhances the yield higher by the order, while large scale makes the number density to approach that of homogeneous fields and the chirping increases the yield 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 in future on enhancing the pair production by the frequency chirping are outlined briefly.

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