From Faraday rotation to “Faraday oscillation”: Coherence effect between real and virtual excitons

This work provides the microscopic mechanisms responsible for circular birefringence changed into linear birefringence when the polarization of the excitons present in a semiconductor sample goes from circular to linear. This change shows up as Faraday rotation turning to “Faraday oscillation”: The probe polarization plane oscillates instead of rotates while the polarization goes from linear to elliptical and linear again. This oscillation, which reduces to zero when the probe polarization is parallel or perpendicular to the exciton polarization, comes from a non trivial coherence effect between real excitons present in the sample and virtual excitons coupled to unabsorbed photons. While Faraday rotation mainly follows from one single carrier exchange between composite excitons in the absence of Coulomb interaction, Faraday oscillation requires two Coulomb interactions plus a double carrier exchange, the virtual excitons coupled to the unabsorbed “in” and “out” photons being made with different electrons and holes, as nicely revealed by Shiva diagrams which visualize the many-body physics taking place in composite-exciton systems.