Study On Electron-positron Pair Production In Vacuum Polarized Strong Fields With Frequency Chirps

At the establishment of Quantum Electrodynamics,it was theoretically predicted that the vacuum would decay into electron-positron pairs in a strong electric field,which is the Sauter-Schwinger effect.But at that time the laser intensity was far less than the critical field intensity.In the early 1980 s,Strickland and Mourou proposed a chirped pulse amplification technique,which led to the rapid development of laser technology and the further improvement of electric field strength.The generation of electron-positron pairs in a vacuum will be confirmed by experiments in the near future.When applying chirp to the theoretical study of the Sauter-Schwinger effect,proper chirp parameters can increase the number of particles produced by several orders of magnitude.Firstly,we report the effect of an electric field with symmetrical frequency chirp on the generation of electron-positron pairs.By using the Dirac-Heisenberg-Wigner method,we have studied the generation of electron-positron pairs in a linearly,elliptically,nearcircularly,and circularly polarized electric field with symmetrical frequency chirp and obtained corresponding momentum spectrum and number density.For small chirp,the difference in results between different polarized electric fields is obvious.When the chirp parameter increases,the momentum spectra tend to exhibit the multiphoton pair generation that is characterized by the multi concentric ring structure.The increase of number density is also remarkable compared to the case of asymmetrical frequency chirp.Note that the dynamically assisted Sauter-Schwinger Mechanism plays an important role for the enhanced pair production in the symmetrical frequency chirp.Secondly,in order to further study the chirped electric field with the same symmetrical pulse shape,we studied the influence of the chirped electric field with symmetrical frequency on the generation of electron-positron pairs.The result of momentum spectra in linearly polarized field shows the expansion and splitting of peaks and strong interference effect with the increase of the chirp parameter.The phenomena are consistent with the result in the linearly polarized electric field with symmetrical frequency chirp.For small chirp,the number density corresponding to the elliptically and circularly polarized electric field is two orders of magnitude higher than the number density of the linearly polarized electric field.In addition,the momentum spectra of circularly and elliptically polarized electric fields with small chirp are extremely similar,while for large chirp,irregular structures appear in the momentum spectra.The above results not only prove the existence of the dynamically assisted Sauter-Schwinger Mechanism,but also shows that the shape of the electric field has a great influence on the generation of electron-positron pairs.