Controlling Subcycle Optical Chirality in the Photoionization of Chiral Molecules

Abstract

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Over the past decades, many strategies have been developed to reveal structural or dynamical information about chiral molecules with high sensitivity, from the microwave to the extreme ultraviolet range. Most schemes employ circularly or elliptically polarized radiation, and more sophisticated configurations involve, for instance, light pulses with time-varying polarization states. All these schemes share a common property—the polarization state of light is always considered as constant over one optical cycle. In this study, we focus on the optical cycle in order to resolve and control a subcyle chiroptical process. We engineer an electric field whose instantaneous chirality can be controlled within the optical cycle, by combining two phase-locked orthogonally polarized fundamental and second harmonic fields. While the composite field has zero net ellipticity, it shows an instantaneous optical chirality which can be controlled via the two-color delay. We theoretically and experimentally investigate the photoionization of chiral molecules with this controlled chiral field. We find that electrons are preferentially ejected forward or backward relative to the laser propagation direction depending on the molecular handedness, similarly to the well-established photoelectron circular dichroism process. However, since the instantaneous chirality switches sign from one half-cycle to the next, electrons ionized from two consecutive half-cycles of the laser show opposite forward-backward asymmetries. This chiral signal, the enantiosensitive subcycle antisymmetric response gated by electric-field rotation, provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. More generally, our results demonstrate the important role of subcycle polarization shaping of electric fields as a new route to study and manipulate chiroptical processes.