Elliptically polarized high-order harmonics generated in aligned CO 2 molecules

Ultrashort extreme ultraviolet (EUV) laser pulses with a controllable polarization state are highly desirable for time-resolved studies and applications involving chirality, circular dichroism, spin dynamics and magnetism in the gas phase or condensed matter. A convenient method to generate such radiation is higher-order harmonic generation (HHG) as applied in numerous laboratories worldwide. Unless special techniques are employed, however, this process will produce predominantly linearly polarized EUV radiation as imposed by the symmetry of the generating field-medium system. To overcome this fundamental limitation, intense research activity has focused on shaping the time-dependent field vector of the generating light pulse [1]. In the current contribution we present an alternative method to generate EUV pulses with a widely controllable polarization state utilizing field-free aligned molecules as an anisotropically shaped medium for HHG.