Inducing elliptically polarized high-order harmonics from aligned molecules with linearly polarized femtosecond pulses

A recent paper reported elliptically polarized high-order harmonics from aligned N-2 using a linearly polarized driving field [X. Zhou et al., Phys. Rev. Lett. 102, 073902 (2009)]. This observation cannot be explained in the standard treatment of the Lewenstein model and has been ascribed to many-electron effects or the influence of the Coulomb force on the continuum electron. We show that nonvanishing ellipticity naturally appears within the Lewenstein model when using a multicenter stationary-phase method for treating the dynamics of the continuum electron. The reason for this is the appearance of additional contributions, which can be interpreted as quantum orbits in which the active electron is ionized at one atomic center within the molecule and recombines at another. The associated exchange harmonics are responsible for the nonvanishing ellipticity and result from a correlation between the ionization site and the recombination site in high-order harmonic generation.