Achiral molecule is a kind of molecule which is not superimposable on its mirror image. The property of chirality is of great practical importance because most biomolecules and pharmaceuticals are chiral and many chemical processes in living beings are chirality dependent. Therefore, detection and recognition of chirality is one of the most important and challenging tasks for researches and applications in the biological and pharmaceutical areas. If a molecule has more than one chiral centers, it can be either a chiral molecule or an achiral molecule. For example, if a molecule has two R-type chiral centers, it is a right-handed chiral molecule. However, if the molecule has two enantiomeric chiral centers which are R-type and S-type respectively, it is an achiral molecule known as a meso isomer. Generally, a chiral molecule possesses circular dichroism in the optical response to circularly polarized light, while the optical responses are the same from a meso molecule. As a result, it is difficult to discriminate the meso compound and racemic mixture (the mixture that has equal amounts of left- and right-handed enantiomers) with conventional optical methods.

The ultrafast laser team lead by Prof. Peixiang Lu in Wuhan National Laboratory for Optoelectronics has studied high harmonic generation (HHG) from isomers of molecule with two chiral centers, taking orientated tartaric acid as prototype, driven by intense circularly polarized femtosecond laser pulses. Anomalous circular dichroism in HHG is found. The obtained HHG spectra driven by left and right circularly polarized laser pulses from the chiral isomers are nearly identical, while the HHG spectra from the meso isomer are significantly different. This effect is even more pronounced with bicircular laser fields. Detailed analysis shows that this anomalous circular dichroism results from the characteristic highly non-perturbative recollision process of HHG. This work proposes a method to discriminate the meso and racemic samples based on HHG.

This work “Anomalous circular dichroism in high harmonic generation of stereoisomers with two chiral centers”was supported by the National Natural Science Foundation of China under Grants No. 11404123, 11234004, 61275126, 11422435, and 11574101. The research result is published in Optics Express 24, 24824-24835 (2016).

(a) Electric field of the applied circularly polarized laser pulse. (b) Comparison between the obtained HHG spectra driven by left and right circularly polarized laser fields from the right-handed chiral isomer denoted as RR-tartaric acid. The inset is the Newman projection of the RR-tartaric acid. (c) Comparison between the obtained HHG spectra driven by left and right circularly polarized laser fields from the meso isomer denoted as RS-tartaric acid. The inset is the Newman projection of the RS-tartaric acid. (d) Sum of the HHG spectra from RR- and SS-tartaric acids.