T-matrix scattering formalism for electron-beam spectroscopy

Advanced computational tools that describe the interaction of electrons with structured nanophotonic devices are crucial for theoretical predictions, specific design tasks, and the interpretation of experimental results. These tools open the door to a systematic exploration of light-matter interactions in complex photonic environments and support the development of next-generation nanophotonic platforms. Here, we report on the implementation of electron-beam spectroscopy in a T-matrix-based scattering formulation. Such a framework is quite versatile in predicting the electromagnetic response of complex photonic materials composed of periodically or aperiodically arranged individual scatterers. By extending this formalism to describe interactions with fast electrons, we provide a fast and accurate numerical tool for simulating cathodoluminescence (CL) and electron energy-loss spectroscopy (EELS) measurements. The desired functionalities are implemented in treams_ebeam , an extension of the existing software suite treams for electromagnetic scattering computations, available at https://github.com/tfp-photonics/treams_ebeam . We demonstrate the implementation details on a carefully selected set of problems, including single scatterers of various shapes and materials, a periodic chain of elliptical nanodisks, and a finite cluster of nanospheres arranged in a two-dimensional (2D) lattice. By uniting fast-electron physics with advanced scattering theory, our framework unlocks new possibilities for designing, understanding, and engineering next‑generation nanoscale light–matter interactions.