Control of ellipticity in high-order harmonic generation driven by two linearly polarized fields

We demonstrate experimentally the control of the polarization (ellipticity and polarization axis) of femtosecond high-order harmonics. The method relies on a two-color ($\ensuremath{\omega}$ -- $2\ensuremath{\omega}$) configuration, where both $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ generating fields have linear polarizations with a variable crossing angle. We correlate our measurements with the conservation rules of energy and linear momentum accounting for harmonic generation in a two-color field. We evidence that the generation process, and especially the number of $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ photons absorbed for generating a given harmonic, strongly depends on the crossing angle. Finally, relying on a simple formalism, we derive analytical formul\ae{} for calculating both polarization axis and ellipticity of the separate harmonics. The model corroborates our results and represents a base for future investigations.