Chiral Light Emission from a Sphere Revealed by Nanoscale Relative Phase Mapping

Circularly polarized light (CPL) is currently receiving much attention as a key ingredient for next-generation information technologies, such as quantum communication and encryption. CPL photon generation for such applications is commonly realized by coupling achiral optical quantum emitters to chiral nanoantennas. Here, we explore a different strategy consisting in exciting a nanosphere -- the ultimate symmetric structure -- to produce all-directional CPL emission. Specifically, we demonstrate chiral emission from a silicon nanosphere induced by an electron beam based on two different strategies: dissolving the degeneracy of orthogonal dipole modes, and interference of electric and magnetic modes. We prove these concepts by visualizing the phase and polarization using a newly developed polarimetric four-dimensional cathodoluminescence method. Besides their fundamental interest, our results support the use of free-electron-induced light emission from spherically symmetric systems as a versatile platform for the generation of chiral light with on-demand control over the phase and degree of polarization

Simultaneous nanoscale excitation and emission mapping by cathodoluminescence

Free-electron-based spectroscopies can reveal the nanoscale optical properties of semiconductor materials and nanophotonic devices with a spatial resolution far beyond the diffraction limit of light. However, the retrieved spatial information is constrained to the excitation space defined by the electron beam position, while information on the delocalization associated with the spatial extension of the probed optical modes in the specimen has so far been missing, despite its relevance in ruling the optical properties of nanostructures. In this study, we demonstrate a cathodoluminescence method that can access both excitation and emission spaces at the nanoscale, illustrating the power of such simultaneous excitation and emission mapping technique by revealing a sub-wavelength emission position modulation as well as by visualizing electromagnetic energy transport in nanoplasmonic systems. Besides the fundamental interest of these results, our technique grants us access into previously inaccessible nanoscale optical properties