Attosecond Optical Orientation

TL;DR

This paper introduces a novel attosecond spectroscopy technique using circularly polarized pulses to optically orient excited states, enabling the study of spin-dependent dynamics without symmetry-breaking conditions.

Contribution

It presents a new method for circular-dichroic attosecond transient absorption spectroscopy that leverages angular momentum of circular pulses in isotropic media, expanding capabilities for probing excited state dynamics.

Findings

Observation of spectral reshaping due to the AC Stark effect.

Identification of selection and propensity rules in the absorption process.

Potential to study spin-dependent dynamics in atoms and molecules.

Abstract

Circularly polarized light offers opportunities to probe symmetry-dependent properties of matter such as chirality and spin. 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 angular momentum of two circular-polarized pulses, both pump and probe, in an isotropic medium, optically orienting the angular momentum of excited states on an attosecond timescale. We investigate a circular-dichroic measurement of the attosecond transient absorption of He Rydberg states. By limiting the allowed pathways via dipole selection rules for co- and counter-rotating circular polarized NIR and XUV pulses, different spectral reshapings of the XUV transient absorption due to the AC Stark effect are observed. Paired with time-dependent Schr\"odinger equation calculations, the results show the role of selection and propensity rules and open up new opportunities to study coupling pathways of excited states as well as spin-dependent dynamics in atoms and beyond via attosecond optical orientation.