Numerical simulations of interference effects in photon-assisted electron energy-loss spectroscopy

Although the electromagnetic resonances of individual nanostructures can be studied by electron or photon interactions alone, exciting new possibilities open up through the simultaneous use of both. In photon-induced near-field electron microscopy (PINEM), for example, single nanostructures are optically excited by short, intense pulses and concurrently imaged with high spatial resolution by fast electrons, which act as negligible probes of electric fields. Controlling their relative arrival time provides access to the dynamics of the electromagnetic response in the near field by recording images of the electron energy loss (or gain) spectra. In this paper, we investigate the transition from optically dominated PINEM to conventional, electron-dominated electron energyloss spectroscopy (EELS). During the systematic reduction of optical excitation intensity to zero, a novel electro-optical interference effect emerges. It reveals itself at those optical field strengths that lead to prominently visible constructive and destructive interference patterns of the optical and electron radiation fields which are scattered by a nanostructure. The interference patterns reported here allow one to achieve higher temporal, energy and spatial resolutions of the modal dynamics in electron microscopy.