摘要： 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. Here we propose and theoretically explore a novel HHG-based chiroptical approach that relies solely on electric-dipole interactions. The method is implemented through bi-chromatic non-collinear HHG, where the beams’ properties are chosen from group theory symmetry-based considerations to exhibit reflection or inversion dynamical symmetries (DSs) [9]. This scheme leads to forbidden harmonic selection rules from isotropic achiral media which are broken in chiral media, because it does not exhibit reflection and inversion symmetries. As a result, ‘forbidden’ harmonics are emitted only if the medium is chiral, and their intensity is correlated to the enantiomeric excess (ee), providing a single-shot background free signal. We analytically derive the general conditions that allow an electric-dipole based chiral response, and numerically demonstrate several feasible geometries [10]. For instance, using DS group theory considerations [9], we numerically demonstrate that the bi-chromatic non-collinear chiral HHG scheme presented in Fig. 1(a) – two intense non-collinear bi-chromatic (3:5 carrier frequency ratios), counter rotating, elliptically polarized beams propagate with a relative opening angle of 2α – leads to an electric-dipole based ‘forbidden harmonic’ signal. When this field interacts with an achiral medium (e.g. a non-oriented racemic mixture of chiral molecules), even harmonic emission is forbidden due to a dynamical inversion symmetry selection rule (the pump is invariant under the DS: (cid:1870)?→-(cid:1870)?, t→t+T/2) [9]. However, when this field interacts with a chiral medium, even harmonics are emitted in all polarizations, and the electric-dipole response does not average-out to zero [10]. This results from the medium breaking the pump’s inversion DS; hence, the intensity of the even harmonics is correlated to the ee, while the odd harmonics are chirality-independent and can be used as a reference (see Fig. 1(c)). This scheme leads to a nearly background free chiral/achiral signal, reaching as high as 97% for the normalized harmonic response from chiral media compared to achiral media (Fig. 1(d)).