Probing unconventional nonlinear optical phenomena with electrons and graphene nanostructures.

June 22, 2020

Optical spectroscopies rely on the response of a sample to the electromagnetic field of an external light source. Electron beams can supply these external fields on sub-Angstrom spots, well-below the diffraction limit of conventional optical setups, where spectral analysis of the electron energy loss or the resulting cathodoluminescence (CL) light emission from their interaction with the sample enables the study of optical excitations with millielectronvolt energy resolution. Electron spectroscopies are understood as probes of the linear optical response, while nonlinear dynamics has thus far escaped observation with a similar level of spatial detail, despite the strong enhancement of the electron evanescent field with decreasing electron energy.

Recently, an international collaboration between D-IAS Assistant Professor Joel Cox at the Center for Nano Optics and ICREA Prof. Javier García de Abajo at The Institute of Photonic Sciences (ICFO) in Barcelona, Spain revealed that low-energy electrons can trigger a substantial nonlinear optical response in the so-called plasmons–collective charge density oscillations—supported in graphene nanoislands [J. D. Cox and F. J. García de Abajo, Nano Lett. (in press, 2020)]. Based on rigorous quantum-mechanical simulations, the nonlinear interactions between free electrons and nanographenes are predicted to give rise to spectral shifts and saturation in the plasmonic fingerprints of CL and electron energy-loss spectroscopies. The proposed electron-induced nonlinearity supports the use of low-energy electron beams for the exploration of nonlinear optical processes on the nanoscale, and warrants further study on the quantum information that can be encoded and transferred between nanoscale optical resonators and individual free electrons.