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 used in those 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 CPL emission along an arbitrary direction. Specifically, we demonstrate chiral emission from a silicon nanosphere induced by an electron beam based on two different strategies: either shifting the relative phase of degenerate orthogonal dipole modes or interfering electric and magnetic modes. We prove these concepts both theoretically and experimentally by visualizing the phase and polarization using a fully 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.