Mirror Optical Activity: Nanophotonic Chiral Sensing from Parity Indefiniteness

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 the chiro-optical effects routinely used to detect enantiomers in mixtures, such as circular dichroism, rely on chiral sensitivity to photon circular polarization, and thus do not harness the full potential of mirror-symmetry breaking, which also involves the spatial profile of the radiation. Here we show that the parity indefiniteness of an electromagnetic field interacting with chiral matter supports mirror optical activity, a chiro-optical effect where a chiral film, when it is 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 can provide an efficient chiral sensing technique. We specialize the discussion to nanofilms with infrared chirality by using a swift electron in an aloof configuration as the nanoemitter and an off-axis transparent conducting nanoparticle as the near-field probe; the spatial dissymmetry factor of the nanoparticle cathodoluminescence is 1 order of magnitude larger than the dimensionless circular dichroism, and is further enhanced to 2 orders of magnitude if an additional graphene sheet is deposited on the film interface.