  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.
