[Paper Review] Controlling Sub-Cycle Optical Chirality in the Photoionization of Chiral Molecules

Shaked Rozen, Antoine Comby+9 more|arXiv (Cornell University)|Jun 26, 2019

Laser-Matter Interactions and ApplicationsPhysics and Astronomy52 references60 citations

TL;DR

The paper combines theory and experiment to shape sub-cycle optical chirality using a two-color field (fundamental plus phase-locked second harmonic) to induce enantiosensitive, sub-cycle photoionization (ESCARGOT) in chiral molecules, revealing forward/backward and up/down asymmetries that depend on two-color delay.

ABSTRACT

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Many strategies have been developed to reveal structural or dynamical information about chiral molecules, 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. In all these schemes, the polarization state of light is always considered as constant over one optical cycle. In this study, we zoom into the optical cycle in order to resolve and control a subcyle attosecond 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 provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. Our results demonstrate the important role of sub-cycle polarization shaping of electric fields, as a new route to study and manipulate chiroptical processes.

Motivation & Objective

• Motivate sub-cycle control of light's instantaneous chirality to probe chiroptical dynamics at attosecond scales.
• Develop and test a two-color (fundamental + phase-locked second harmonic) field to create instantaneous chirality with zero net ellipticity.
• Demonstrate experimentally and theoretically that instantaneous chirality drives enantiosensitive photoionization signals (ESCARGOT).
• Explore both multiphoton and strong-field ionization regimes and connect with conventional PECD concepts.

Proposed method

• Engineer a two-color electric field E(t)=E0 cos(ωt) ŷ + rE0 cos(2ωt+φ) x̂ with r≈0.3.
• Define instantaneous ellipticity ε(t) and instantaneous chirality C(t) from E(t) and B(t) to quantify sub-cycle field properties.
• Compute and simulate photoionization by solving the time-dependent Schrödinger equation (TDSE) for a toy chiral molecule in the bichromatic field, extracting ionized parts and momentum-space distributions.
• Rotate between molecular and lab frames using Euler rotations and map to VMI momentum distributions to compare with experiment.
• Decompose the VMI distributions into symmetric and antisymmetric parts with respect to the propagation axis to isolate ESCARGOT signals.

Experimental results

Research questions

• RQ1 Can instantaneous chirality within an optical cycle drive enantiosensitive signals in photoionization when the net field chirality is zero over a cycle?
• RQ2 How does the ESCARGOT signal depend on two-color delay φ and laser intensity across multiphoton and strong-field regimes?
• RQ3 What is the role of field shape (C-shaped vs 8-shaped) in generating enantiosensitive sub-cycle responses?
• RQ4 To what extent do experimental measurements in fenchone and camphor agree with TDSE-based toy-model predictions about sub-cycle chiral effects?

Key findings

• ESCARGOT signals (antisymmetric photoelectron distributions) appear for all two-color delays and in all ionization regimes studied.
• In multiphoton regimes, ESCARGOT reaches ~1.5% and exhibits structured angular patterns with alternating signs.
• In the strong-field regime, ESCARGOT persists but decreases to ~0.5% and sharpens near the polarization plane, with ring structures linked to ATI features.
• C-shaped fields can produce significant forward/backward up/down asymmetries, due to the temporal derivative of the field (vector potential) imparting opposite rotations in consecutive half-cycles.
• Experimental fenchone data show ESCARGOT signals consistent with TDSE predictions, including phase-dependent sign changes, while camphor results reveal molecule-specific differences and less agreement with the toy model.
• The work highlights the sensitivity of sub-cycle chiral photoionization to instantaneous field chirality and demonstrates a new route to control chiroptical processes.