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.
- Received 20 December 2018
- Revised 16 April 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031004
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Focus
FocusMolecular Probe Uses a Polarization Flip
Published 8 July 2019
A new way of probing molecules with handedness involves a light pulse in which the polarization changes in the middle of a single wave cycle.
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Popular Summary
Circularly polarized light—in which the electric field of the light traces out a circle as the light propagates—is a powerful tool for studying chirality. This is a fundamental property of matter in which some entity (such as a molecule) exists in two different mirror forms, like left and right human hands. Ultrafast lasers now make it possible to sculpt electromagnetic fields that trace out patterns more complex than a circle, for example, a figure eight, by inverting the rotation direction of the electric field every few hundred attoseconds. Here, we experimentally show that such manipulation of light is a highly sensitive probe of evolving electronic chiral effects in matter on ultrashort timescales.
We set out to study two questions: Is chiral light-matter interaction sensitive to the instantaneous rotation of the electromagnetic field, and will the effects from consecutive opposite lobes of the figure eight cancel out? To answer these questions, we ionize chiral molecules using a sculpted light field and detect the angular distribution of the ejected electrons.
When circularly polarized light photoionizes chiral molecules, electrons are ejected forward or backward, depending on the rotation direction of the light. In our experiments, we observe that electrons ejected towards the top of the figure eight are sent forward, while electrons emitted towards the bottom are sent backward. This indicates that the electrons have been produced by consecutive lobes of the figure eight with opposite chiralities and that the asymmetric chiral light-matter interaction has been imprinted in a few hundreds of attoseconds—a time shorter than a full cycle of the figure-eight field.
The sensitivity of the chiral light-matter interaction to the instantaneous rotation of the electric field thus opens a route to measure and control electronic chiroptical processes on their natural attosecond timescale.
