Electric-Dipole Based Chiral Sensitivity in High Harmonic Generation by Dynamical Symmetry Breaking Spectroscopy

Abstract

Chirality is a fundamental asymmetry property that appears abundantly in nature[1]. A system is chiral if and only if it is distinct from its mirror image (its opposite handedness chiral-partner), e.g. circularly polarized light or chiral molecules. Such systems are unique in that their properties are completely independent of their handedness up until the moment they interact with another chiral object. For instance, partner chiral molecules have identical cross-sections for absorption of linearly-polarized light, but not for absorption of circularly polarized light, leading to circular dichroism (CD)[1]. Standardly, chirality is analyzed by chiroptical techniques that measure the medium's response to elliptically polarized light. However, such techniques rely on magnetic-dipole or higher electric-moment transitions, because electric-dipole interactions average-out to zero in isotropic media (circularly polarized light has a helical pitch that is negligible in the dipole approximation)[1]. Consequently, standard chiroptical approaches lead to very weak signals, especially in the gas phase. In recent years, several seminal electric-dipole based methods were developed that lead to much larger chiral signals, including photoelectron CD[1−4], coulomb explosion imaging[5], and microwave three-wave mixing[6]. Importantly, high harmonic generation (HHG) was shown to be chirality-sensitive, leading to relatively large (up to 10%) femtosecond-resolved chiral signals[7,8]. Still, the chiral signal in HHG is based on magnetic-dipole interactions (same as in the standard techniques), and the signal is relatively large only due to the non-perturbative nature of the process. Extending HHG to produce an electric-dipole chiral response could open-up many possibilities for optically exploring ultrafast chirality and weakly-chiral systems.