[Paper Review] Highly selective chiral discrimination in high harmonic generation by dynamical symmetry breaking spectroscopy

Ofer Neufeld, Oren Cohen|Figshare|Jul 7, 2018

Spectroscopy and Quantum Chemical StudiesPhysics and Astronomy57 references20 citations

TL;DR

This paper proposes a highly selective, single-shot chiral discrimination method in high harmonic generation (HHG) using dynamical symmetry breaking in chiral media. By exploiting intensity differences in symmetry-forbidden harmonics—directly linked to enantiomeric excess—while using allowed harmonics as a chiral-insensitive reference, the method achieves 96% discrimination in gas-phase simulations, outperforming prior time-resolved techniques.

ABSTRACT

We propose and numerically demonstrate a new very robust and highly selective method for femtosecond time-resolved chiral spectroscopy using high harmonic generation (HHG). The method is based on dynamical symmetry breaking from chiral media, and relies only on intense electric-dipole transitions, and not on the interplay of electric and magnetic dipoles. The symmetry breaking results in the emission of a strong chiral signal in the form of otherwise 'forbidden' harmonics (i.e., that are not emitted from achiral media). The intensity of these symmetry-forbidden harmonics is directly correlated to the media's enantiomeric excess, yielding chiral selectivity. On the contrary, the strength of the 'allowed' harmonics is chiral-independent, hence they can be used as a reference to provide chiral selectivity from a single measurement, unlike previous time-resolved schemes that require multiple measurements. We demonstrate numerically 96% discrimination level from microscopic gas phase emission, outperforming by far previous time-resolved methods (the selectivity should be further enhanced when the HHG process is phase matched). We expect the new method to give rise to precise table-top characterization of chiral media in the gas-phase, and for highly sensitive time-resolved ultrafast probing of dynamical chiral processes.

Motivation & Objective

• To develop a robust, single-shot method for ultrafast chiral discrimination in the gas phase.
• To overcome limitations of prior time-resolved chiral spectroscopy requiring multiple measurements.
• To enable precise, real-time probing of dynamical chiral processes using high harmonic generation.
• To achieve high selectivity based solely on electric-dipole transitions without relying on magnetic dipole interactions.
• To demonstrate that symmetry-forbidden harmonics serve as a direct, quantitative probe of enantiomeric excess.

Proposed method

• The method exploits dynamical symmetry breaking in chiral media under intense few-cycle laser excitation.
• It relies on the generation of otherwise 'forbidden' high harmonics due to chiral-induced breaking of spatial inversion symmetry.
• The intensity of these symmetry-forbidden harmonics correlates directly with the enantiomeric excess of the chiral medium.
• Simultaneously, 'allowed' harmonics—chiral-independent—serve as an internal reference to normalize signals and enable single-shot quantification.
• The approach uses only electric-dipole transitions, avoiding the complexity and ambiguity of magnetic dipole contributions.
• Numerical simulations are performed on microscopic gas-phase systems to validate the chiral selectivity and signal-to-noise characteristics.

Experimental results

Research questions

• RQ1 Can dynamical symmetry breaking in chiral media generate a measurable, selective signal in high harmonic generation?
• RQ2 Can symmetry-forbidden harmonics serve as a direct probe of enantiomeric excess in a single measurement?
• RQ3 How does the selectivity of this method compare to existing time-resolved chiral spectroscopy techniques?
• RQ4 To what extent can the signal from symmetry-forbidden harmonics be distinguished from background and noise in realistic gas-phase systems?
• RQ5 Can the method achieve high selectivity without requiring phase matching or complex interferometric setups?

Key findings

• The method achieves a 96% discrimination level between enantiomers in numerical simulations of gas-phase chiral molecules.
• Symmetry-forbidden harmonics emerge strongly only in chiral media, providing a direct and selective signal of enantiomeric excess.
• The intensity of allowed harmonics remains unchanged across enantiomers, enabling their use as a robust internal reference.
• The technique is inherently single-shot, eliminating the need for multiple measurements or complex temporal gating.
• The selectivity is expected to improve further when high harmonic generation is phase-matched in macroscopic media.
• The method relies solely on electric-dipole transitions, simplifying the experimental implementation and enhancing robustness.