Abstract
We propose and numerically demonstrate a new chiral spectroscopy method that is based on a universal system-independent mechanism of dynamical symmetry breaking in high harmonic generation (HHG). The proposed technique relies only on intense electric-dipole transitions and not on their interplay with magnetic dipole transitions. The symmetry breaking results in the emission of otherwise “forbidden” harmonics from chiral media (i.e., that are not emitted from achiral or racemic media), yielding a huge, nearly background-free, chiral-achiral signal that is correlated to the magnitude of the medium’s enantiomeric excess. The handedness of the medium can be directly detected by measuring the polarization helicity of the emitted harmonics. Moreover, the strength of the “allowed” harmonics (that are not related to symmetry breaking) is chirality independent; hence, they can be used as a reference to probe chiral degrees of freedom within a single measurement. We numerically demonstrate up to 99% chiral-achiral signal level (normalized difference between the chiral and achiral HHG spectra) from microscopic gas-phase emission using state-of-the-art models for HHG in bromochlorofluoromethane and propylene oxide. We expect the new method to give rise to precise tabletop characterization of chiral media in the gas phase and for highly sensitive time-resolved probing of dynamical chiral processes with femtosecond-to-attosecond temporal resolution.
- Received 31 May 2018
- Revised 15 March 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031002
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)
Popular Summary
A system is chiral if it cannot be superposed onto its mirror image (its opposite-handedness chiral partner). This fundamental property of nature appears in systems such as circularly polarized light and mirror-image pairs of molecules. Chiral systems are unique in that their properties are independent of their handedness, up until the moment they interact with another chiral object. These properties make chirality extremely important in many scientific fields such as physics, chemistry, biology, and medicine. However, chirality is still very difficult to characterize. Standard detection methods lead to very weak signals, especially in gases. Using numerical simulations, we propose a new way of detecting chirality that relies solely on electric dipole transitions, which should yield an enormous and clear signal.
Our method relies on fundamental symmetry considerations in nonlinear optics, where an intense noncollinear laser shines on a chiral medium. The nonlinear interaction between the laser and the medium emits new photons, whose intensity and polarization can be used to reconstruct the medium’s chirality in ultrafast measurements with subfemtosecond resolution.
We expect that our work will pave the way for highly sensitive chiral measurements and for ultrafast spectroscopy of chiral phenomena, including weakly chiral systems.
