  Multiphoton pair production is investigated by focusing on the momentum
structures of produced pairs in the polarization plane for the two circularly
polarized fields. Upon the momentum spectra, different from the concentric
rings with the familiar Ramsey interference fringes for the same handedness,
however, the obvious vortex structures are found constituted by the Archimedean
spirals for two opposite handedness fields. The underlying physical reasons are
analyzed and discussed. It is also found that the vortex patterns are sensitive
to the relative carrier envelope phase, the time delay, and the handedness of
two fields, which can be used to detect the applied laser field characteristics
as a probe way.
