  The QED cascade induced by the two counter-propagating lasers is studied. It
is demonstrated that the probability of a seed-photon to create a pair is much
larger than that of a seed-electron. By analyzing the dynamic characteristics
of the electron and positron created by the seed-photon, it is found that the
created particles are more probable to emit photons than the seed-electron.
With these result, further more, we also demonstrate that the QED cascade can
be easier to be triggered by the seed-photon than by the seed-electron with the
same incident energy and the same laser.
