Elliptically polarized high-order harmonic emission from molecules in linearly polarized laser fields.

We perform an accurate polarimetry measurement of high-order harmonic emission from aligned molecules. We find that harmonic emission from N2 can be strongly elliptically polarized even when driven by linearly polarized laser fields. These data have broad implications for understanding molecules in strong fields because they cannot be explained by simple theories based on the strong field approximation and single active electron models. Finally, this work also shows that it is possible to engineer the polarization properties of harmonic emission by carefully preparing a molecular medium.