Chiral Discrimination through Bielliptical High-Harmonic Spectroscopy

Abstract

Molecular chirality plays an essential role in most biochemical processes. The observation and quantification of chirality-sensitive signals, however, remains extremely challenging, especially on ultrafast timescales and in dilute media. Here, we describe the experimental realization of an all-optical and ultrafast scheme for detecting chiral dynamics in molecules. This technique is based on high-harmonic generation by a combination of two-color counterrotating femtosecond laser pulses with polarization states tunable from linear to circular. We demonstrate two different implementations of chiral-sensitive high-harmonic spectroscopy on an ensemble of randomly oriented methyloxirane molecules in the gas phase. Using two elliptically polarized fields, we observe that the ellipticities maximizing the harmonic signal reach up to 4.4 AE 0.2% (at 17.6 eV). Using two circularly polarized fields, we observe circular dichroisms ranging up to 13 AE 6% (28.3-33.1 eV). Our theoretical analysis confirms that the observed chiral response originates from subfemtosecond electron dynamics driven by the magnetic component of the driving laser field. This assignment is supported by the experimental observation of a strong intensity dependence of the chiral effects and its agreement with theory. We moreover report and explain a pronounced variation of the signal strength and dichroism with the driving-field ellipticities and harmonic orders. Finally, we demonstrate the sensitivity of the experimental observables to the shape of the electron hole. This technique for chiral discrimination will yield femtosecond temporal resolution when integrated in a pump-probe scheme and subfemtosecond resolution on chiral charge migration in a self-probing scheme.

Ultrasensitive Chiral Spectroscopy by Dynamical Symmetry Breaking in High Harmonic Generation

“…To summarize, we proposed and numerically demonstrated a novel system-independent approach for cHHG that is based on dynamical symmetry-breaking spectroscopy (and not on the detailed mechanism of the HHG process). Our approach relies solely on intense electricdipole transitions and not on the interplay of both magnetic and electric dipoles as in all other cHHG schemes [28][29][30][31]33], making it strong and highly robust. We explored different feasible realizations for the scheme by systematically engineering pump fields with various dynamical symmetry properties that are broken in chiral media.…”

“…Recently, it was also discovered that high harmonic generation (HHG) with a helically polarized pump is chirality sensitive [28][29][30][31][32][33]. In chiral HHG (cHHG), a chiral medium is irradiated with an intense laser field, and the emission spectrum is measured.…”

“…cHHG is a very promising technique since it naturally leads to all-optical femtosecond time-resolved signals, and it is accessible in the gas phase by simple tabletop setups [28]. However, current implementations of cHHG exhibit relatively low sensitivity: about 3% with a somewhat elliptically polarized driver [28] and about 10% with the so-called bicircular driver [30,31,33]). This relatively low sensitivity means, for example, that if the enantiomeric excess of the medium is 20%, then the observed discrimination is about 2%, which may already be within the noise level and therefore not detectable.…”

Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

“…Being able to use an industrial nanosecond laser to investigate PECD furthermore reduces the technical requirements to apply PECD in analytical chemistry. Chiral recognition in the gas phase with the help of electromagnetic radiation is an emerging research area as an interaction-free environment allows to study intrinsic chiral response.In the last few years for instance fully coherent microwave three-wave mixing schemes, [1] Coulomb explosion imaging for direct absolute configuration determination, [2] circular dichroism (CD) in all-optical high-harmonic spectroscopy [3] as well as CD in ion yield [4] have been developed.When chiral molecules are ionized by circularly polarized light, a forward-backward asymmetry in the photelectron emission direction with respect to the light propagation direction was predicted by theory. [5] This effect arises in the electric dipole approximation and was first observed experimentally using single-photon ionization.…”

A dynamical symmetry triad in high-harmonic generation revealed by attosecond recollision control

“…In the case of high-order harmonic generation (HHG), the separate emission events represent recollisions of an electronic wavepacket with an atomic or molecular parent ion [29,30]. The recollision picture facilitates the retrieval of attosecond timing and orientation from spectral features in HHG, as exploited in measurements of the time structure of the tunneling process [31,32], molecular tomography [33], and chiral discrimination [34,35].…”