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

TL;DR

This paper demonstrates that elliptically polarized high-order harmonics from aligned molecules can be explained within the Lewenstein model by considering multi-center quantum orbits, revealing a new mechanism for polarization control.

Contribution

It introduces a multi-center stationary phase approach within the Lewenstein model to explain elliptically polarized harmonics in aligned molecules, challenging previous explanations.

Findings

Ellipticity arises naturally in the Lewenstein model with multi-center analysis.

Exchange harmonics from different atomic centers cause non-zero ellipticity.

Correlation between ionization and recombination sites explains polarization properties.

Abstract

A recent paper reported elliptically polarized high-order harmonics from aligned N$_2$ using a linearly polarized driving field [X. Zhou \emph{et al.}, Phys. Rev. Lett. \textbf{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 non-vanishing ellipticity naturally appears within the Lewenstein model when using a multi-center stationary phase method for treating the dynamics of the continuum electron. The reason for this is the appearance of additional contributions, that 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 non-vanishing ellipticity and result from a correlation between the ionization site and the recombination site in high-order harmonic generation.