Disentangling cathodoluminescence spectra in nanophotonics: particle eigenmodes vs transition radiation

TL;DR

This paper demonstrates that transition radiation significantly influences cathodoluminescence spectra in nanophotonics, and provides methods to distinguish it from particle eigenmodes for accurate spectral interpretation.

Contribution

It introduces a combined experimental and theoretical approach to differentiate transition radiation from eigenmodes in cathodoluminescence spectra of nanostructures.

Findings

Transition radiation can produce distinct spectral resonances.

Interference effects depend on electron beam flight time.

The method improves spectral interpretation accuracy.

Abstract

Cathodoluminescence spectroscopy performed in an electron microscope has proven a versatile tool for analysing the near- and far-field optical response of plasmonic and dielectric nanostructures. Nevertheless, the transition radiation produced by electron impact is often disregarded in the interpretation of the spectra recorded from resonant nanoparticles. Here we show, experimentally and theoretically, that transition radiation can by itself generate distinct resonances which, depending on the time of flight of the electron beam inside the particle, can result from constructive or destructive interference in time. Superimposed on the eigenmodes of the investigated structures, these resonances can distort the recorded spectrum and lead to potentially erroneous assignment of modal characters to the spectral features. We develop an intuitive analogy that helps distinguish between the two contributions. As an example, we focus on the case of silicon nanospheres, and show that our analysis facilitates the unambiguous interpretation of experimental measurements on Mie-resonant nanoparticles.