Dynamically tuneable helicity in twisted electromagnetic resonators

Abstract

We report the generation of helical electromagnetic radiation in a microwave cavity resonator, achieved by introducing mirror asymmetry, i.e., chirality, through a controlled geometric twist of the conducting boundary conditions. The emergence of electromagnetic helicity is attributed to a nonzero spatial overlap between the electric and magnetic mode eigenvectors, quantified by $\text{Im}\left[{\vec{\mathbf{E}}}_i(\vec{r})\cdot {\vec{\mathbf{H}}}_i^{\ast }(\vec{r})\right]$, a feature not observed in conventional cavity resonators. This phenomenon originates from magnetoelectric coupling between nearly degenerate transverse electric (TE) and transverse magnetic (TM) modes, resulting in a measurable frequency shift of the resonant modes as a function of the twist angle, $\ddot{I}•$. In addition to the bulk helicity induced by global geometric twist, internal helical corrugations break structural symmetry on the surface, introducing an effective surface chirality $\hat{I}{º}_{\text{eff}}$, which perturbs the resonant conditions and contributes to asymmetric frequency tuning. By dynamically varying $\ddot{I}•$, we demonstrate real-time, macroscopic manipulation of both electromagnetic helicity and resonant frequency. Furthermore, we investigate the underlying mode-coupling dynamics of the system, highlighting strong photon-photon interactions.

11 pages, 12 figures