Circular dichroism in attosecond transient absorption spectroscopy of isotropic media

Circular polarized light offers opportunities to probe symmetry-dependent properties of matter such as chirality, inversion-symmetry and spin. However, circular dichroism measurements typically require further intrinsic or extrinsic breaking of symmetry by e.g. enantiomeric excess, orientation, magnetic fields or direction-sensitive detectors. Here we introduce circular-dichroic attosecond transient absorption spectroscopy by leveraging the spin-selectivity of two circular-polarized pulses, both pump and probe, in an isotropic medium, breaking the symmetry by preparing spin-specific excited states. As a proof-of-principle we demonstrate a circular-dichroic measurement of the attosecond transient absorption of He Rydberg states: By limiting the allowed coupling pathways due to magnetic quantum number selection rules for co- and counter-rotating circular polarized NIR and XUV pulses, different spectral reshaping of the XUV absorption due to the AC Stark effect is observed. Paired with time-dependent Schrödinger equation calculations, our results allow for an in-situ determination of the XUV ellipticity in an absorption experiment. Our results open up new opportunities to study coupling pathways of excited states as well as spin-dependent dynamics.