Inelastic Scattering of Electron Beams by Nonreciprocal Nanotructures

TL;DR

This paper demonstrates that electron energy-loss spectroscopy and cathodoluminescence can detect direction-dependent optical excitations in nonreciprocal nanostructures, with tunability via magnetic fields, advancing high-resolution probing of such systems.

Contribution

It provides a theoretical analysis of electron-beam interactions with nonreciprocal materials, revealing direction-dependent spectral features and tunability using magnetic bias.

Findings

Spectral features depend on electron propagation direction.

Magnetic field tuning modifies optical resonances.

Direction-dependent coupling observed in EELS and CL.

Abstract

Probing optical excitations with high resolution is important for understanding their dynamics and controlling their interaction with other photonic elements. This can be done using state-of-the-art electron microscopes, which provide the means to sample optical excitations with combined meV--sub-nm energy--space resolution. For reciprocal photonic systems, electrons traveling in opposite directions produce identical signals, while this symmetry is broken in nonreciprocal structures. Here, we theoretically investigate this phenomenon by analyzing electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) in structures consisting of magnetically biased InAs as an instance of gyrotropic nonreciprocal material. We find that the spectral features associated with excitations of InAs films depend on the electron propagation direction in both EELS and CL, and can be tuned by varying the applied magnetic field within a relatively modest sub-tesla regime. The magnetic field modifies the optical field distribution of the sampled resonances, and this in turn produces a direction-dependent coupling to the electron. The present results pave the way to the use of electron microscope spectroscopies to explore the near-field characteristics of nonreciprocal systems with high spatial resolution.