Circularly polarized high harmonic generation through virtual circular birefringence

We suggest a macroscopic scheme for generating circularly polarized high harmonics. The scheme uses a superposition of circularly polarized pump beams with different spatial modes, realizing a perturbed linearly polarized field which rotates along the propagation axis due to virtual circular birefringence. This method provides a spectral selectivity for the harmonic orders which are generated efficiently and for their spacing. The scheme is investigated numerically with a good agreement with the theory.