Cathodoluminescence spectroscopy (CL) is a unique technique to coherently excite and probe optical modes at the nanoscale. So far, CL has probed the angle-dependent spectrum and polarization of nanoscale emitters. However, detecting the phase of the emitted wavefronts has remained elusive.
Here, we introduce Fourier-transform CL holography as a method to determine the far-field phase distribution of scattered plasmonic fields. We measure the interference between: (1) the electron-induced CL emitted by a plasmonic nanoscatterer and (2) a broadband reference field created by transition radiation induced by the same electron. From the angular interference patterns we directly reconstruct angle-resolved phase and intensity distributions. Taking the 6 (x-y-z) plasmonic electric and magnetic dipoles as a complete orthogonal set of scatterers we directly derive from these data the relative strength and phase of all scattering dipoles, as they are excited through the electron beam.
We investigate the resonant scattering of 30 keV electron-beam excited surface plasmon polaritons (SPPs) off single-crystalline Ag nanocubes and find dominant scattering from the z-oriented electric dipole plasmon. In contrast, SPP scattering from nanoscale holes made in a Ag film induces an in-plane x-oriented electric dipole with the concomitant y-oriented magnetic dipole. Using a newly developed CL energy-momentum spectroscopy configuration we also derive the phase of scattered fields as a function of frequency. The data are fully consistent with the plasmon polariton dispersion and the pi phase flip across the scattering resonance is directly observed in the measured phase fronts.
Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies at nanoscale spatial resolution. It also opens up novel ways to investigate the temporal and spatial coherence of electron beam wavefonts and addresses fundamental questions regarding the collapse of the electron wavefunction as it excites surface plasmons.