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 (ω - 2ω) configuration, where both ω and 2ω 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 ω and 2ω photons absorbed for generating a given harmonic, strongly depends on the crossing angle. Finally, relying on a simple formalism, we derive analytical formulæ for calculating both polarization axis and ellipticity of the separate harmonics. The model corroborates our results and represents a base for future investigations.