Infrared Polarization Properties of an Electron-Stimulated Magnetic Dipole Resonance

Abstract

Understanding the electromagnetic properties of the subwavelength building blocks of optical metamaterials requires analysis techniques capable of probing the multimodal response on a highly localized scale, which is particularly challenging in the infrared. In this work, we use cathodoluminescence in a scanning electron microscope to reveal the polarization features of a localized surface plasmon resonance mode in the near infrared, demonstrating that the relative contributions of internal electromagnetic moments can be determined via quantitative analysis of the emission. We analyze the response of a three-dimensional nanoscale split-ring resonator, a structure that is important to metamaterial applications due to its unique ability to support degenerate orthogonal electric and magnetic dipole moments in the lowest-order resonance mode. The interference of far-field radiation from this resonance results in a significant amount of circular polarization, which we use to derive the relative phase shift and field magnitude contributions of each individual moment via comparison with simulations and analytical radiation fields. We find that the magnetic dipole moment contributes to a significant fraction, approximately one-third, of the emitted field strength. Furthermore, the location of the excitation probe and its effect on the net symmetry of the system are of high importance in determining the polarization state of the observed emission.