Sub-half-cycle polarization gates in ultra-short laser pulses induced by non-linear propagation effects

We demonstrate the possibility of imprinting a controlled ultra-fast time-varying ellipticity on few-cycle laser pulses through the non-linear propagation in a thin anisotropic crystal. The method is based on the self- and cross-phase modulation induced by the third-order (Kerr) non-linearity when the pulse propagates through the crystal. The feasibility of this technique is demonstrated through the numerical integration of the Maxwell equations, showing the possibility of producing a sub-half-cycle temporal gate of linear polarization. In comparison with alternative approaches, our method results in a polarization gate with higher efficiency, and also it can be controlled by changing the polarization axis of the laser pulse at the entrance of the crystal. We finally demonstrate the efficiency of this scheme in the generation of single attosecond pulses.