Mirror optical activity: efficient chiral sensing from electromagnetic parity indefiniteness

Abstract

Mirror symmetry is among the most fundamental concepts of physics and its spontaneous breaking at the molecular level allows chiral molecules to exist in two enantiomers that are mirror images of each other. The majority of chiro-optical effects routinely used to detect enantiomers in mixtures, as circular dichroism, relies on chiral sensitivity to photon circular polarization, thus not harnessing the full potentials of mirror symmetry breaking which also involves the radiation spatial profile. Here we show that the parity indefiniteness of the electromagnetic field interacting with chiral matter supports mirror optical activity, a novel chiro-optical effect where a chiral film, once probed by the mirror symmetric field of a nanoemitter, produces a near field whose spatial profile has broken mirror symmetry. The detection of near field dissymmetry provides an highly efficient chiral sensing technique, thus opening novel avenues to devising nanonophotonic schemes for ultra-efficient chiral discrimination of picogram quantities of molecules. We specialize the technique to nano-films with infrared chirality by using a swift electron in aloof configuration as the nanoemitter and an off-axis transparent conductor nanoparticle as the near field probe; the spatial dissymmetry factor of nanoparticle cathodoluminescence is one order of magnitude larger than circular dichroism, which is further enhanced to two orders if an additional graphene sheet is deposited on the film interface.