Electrodynamics in the near-field regions of anisotropic nanoscopic films and platelets

Abstract

An electromagnetic theory of valence electron excitations by an external electron beam in the near field regions of uniaxial anisotropic nanoplatelets is presented. It is shown that in an interface of high symmetry (i.e., perpendicular to the symmetry axis) only extraordinary waves can be excited by the electron beam, usually as surface-plasmon-polariton modes. However, the anisotropy also allows excited extraordinary waves to propagate as waveguide modes. In an interface of low symmetry a mixture of both waves is inseparably excited. Application is made to directional near-field electron energy loss spectroscopy of uniaxial dielectric nanoplatelets. It is found that all relevant length parameters in this spectroscopy happen to fall in the same range, giving rise to enhanced sensitivity of the electron energy loss (EEL) signal to the size and geometry of the detected nanoparticle. The breakdown of momentum conservation in the electron-plasmon scattering event, associated with the finite size of the platelet along the beam direction, strongly changes the EEL signal pattern.