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.