# Transition radiation in photonic topological crystals: quasi-resonant excitation of robust edge states by a moving charge

## TL;DR

This paper explores how a moving charge can excite topologically protected edge states in photonic crystals through transition edge-wave radiation, with theoretical and experimental validation, opening new avenues for particle acceleration and detection.

## Contribution

It introduces the concept of transition edge-wave radiation in photonic topological insulators and demonstrates its ability to generate valley- and spin-polarized edge states.

## Key findings

- Experimental evidence of edge wave excitation by a moving charge.
- TER enables polarization control of topological edge states.
- Theoretical model matches experimental observations.

## Abstract

We demonstrate, theoretically and experimentally, that a traveling electric charge passing from one photonic crystal into another generates edge waves -- electromagnetic modes with frequencies inside the common photonic bandgap localized at the interface -- via a process of transition edge-wave radiation (TER). A simple and intuitive expression for the TER spectral density is derived and then applied to a specific structure: two interfacing photonic topological insulators with opposite spin-Chern indices. We show that TER breaks the time-reversal symmetry and enables valley- and spin-polarized generation of topologically protected edge waves propagating in one or both directions along the interface. Experimental measurements at the Argonne Wakefield Accelerator Facility are consistent with the excitation and localization of the edge waves. The concept of TER paves the way for novel particle accelerators and detectors.