Chirality is a fundamental property in nature. It manifests in various physical, chemical, and biological processes. Chiral molecules do not have a symmetry plane and exist in pairs of left- and right-handed enantiomers. Chiral molecules show strong enantiomeric selectivity. For example, most isomers of chiral drugs exhibit marked differences in biological activities. Therefore, it is vital to identify the molecular chirality. Furthermore, detection of the chiral dynamics in the femtosecond and sub-femtosecond scales can help people understand the underlying physical mechanisms of the chemical reactions.
With the development of ultrafast laser technology, high-order harmonic spectroscopy provides a powerful tool to explore the molecular structure and electron dynamics. In 2015, a collaborative paper by the Université de Toulouse et al. first uses high-harmonic generation from a randomly oriented gas of molecules to probe molecular chirality. However, this work is based on electric–magnetic interaction and generates weak chiral signals leading to low sensitivity. Since 2019, collaborative papers by Technion-Israel Institute of Technology and Max-Born Institute et al. propose several methods based on dynamical symmetry breaking in high harmonic generation exhibiting strong chiral signals and high enantio-sensitivity. However, nearly all of them depend on the non-collinear superposition of multi-color beams, which requires a highly stable coincidence of beams in time and space in the experiment.
Recently, the Ultrafast Optics Laboratory from Huazhong University of Science and Technology led by Professor Peixiang Lu shows that a structured beam generated by an intense linearly polarized single-color beam can yield strong chiral signals in the high harmonic spectrum. This method provides a simpler and more compact experimental setup and shows a background-free and highly sensitive chirality detection. Figure 1 is the schematic of the experimental setup.
Fig. 1 Schematic of the experimental setup
Research results were published in Chinese Optics Letters Vol. 20, Issue 10 in 2022 under the title of "Yuhang Chen, Peixiang Lu, et al. Background-free detection of molecular chirality using a single-color beam [Invited]", and was selected as the cover.
The study shows that the linearly polarized single-color beam focused by a lens and a prism will generate the field being elliptically polarized in the plane of propagation, as shown in Fig. 2.
Fig. 2 The structured laser beam calculated in COMSOL
Because the ensemble of randomly oriented chiral molecules lacks mirror symmetry, the selection rules of high harmonic generation are different between the chiral and achiral (racemic) ensembles driven by such a structured beam. The symmetry analysis shows that even harmonics polarized perpendicular to the plane of propagation and odd harmonics polarized parallel to the plane of propagation will emit from chiral ensembles. But for the achiral (racemic) ensembles, only odd harmonics polarized parallel to the plane of propagation emit and even harmonics are forbidden. The results are confirmed by numerical calculations by three-dimensional time-dependent density functional theory (TDDFT), as shown in Fig. 3. Since chiral and achiral signals are completely separated in harmonic order, the chiral signals (even harmonics) are background-free. This leads to extremely high sensitivity to the chirality of the target media. Further, the researchers studied the far-field signals, and the results still remain.
Fig. 3 Numerically calculated HHG emissions polarized (a) perpendicular and (b) parallel to the plane of propagation from the chiral and racemic ensembles based on TDDFT
Summary and Outlook
In summary, this work opens the path to background-free and highly sensitive chirality detection using a simple and compact experimental setup. In the follow-up research, the researchers will use this method to study ultrafast chiral dynamics, such as the isomerization of chiral molecules. They will also expand this scheme to liquid and condensed phases and complete the corresponding experiment.