Attosecond Transient Polarimetry
We introduce elliptical-dichroic attosecond transient absorption spectroscopy (eDATAS), an ultrafast attosecond spectroscopy technique in which an atomic gas is probed by elliptically polarized attosecond and near-infrared pulses with an arbitrary angle between the polarization ellipses and a varying time-delay. Using perturbation theory, we show that the transient absorption signal factorizes into form factors whose dependence on pulse ellipticities and polarization-ellipse orientations are governed entirely by dipole selection rules: ellipticity sets the amplitudes of the interfering multiphoton pathways, while the relative orientations of the ellipses control the phase. We reinterpret circular- and linear-dichroic attosecond transient absorption spectroscopy as limiting cases of eDATAS, and validate the theory through an eDATAS measurement in helium, finding excellent agreement. Finally, we demonstrate that eDATAS can also serve as an in situ measurement of the polarization state of the XUV field, avoiding the need for a dedicated XUV polarimeter.