Electromagnetic Helicity in Twisted Cavity Resonators

Abstract

Through left- or right-handed twisting, we investigate the impact of mirror-asymmetry (chirality) of the conducting boundary conditions of an equilaterial triangular cross-section electromagnetic resonator. We observe the generation of eigenmodes with non-zero electromagnetic helicity as a result of the coupling of near degenerate TE${}_{11(p+1)}$ and TM${}_{11p}$ modes. This can be interpreted as an emergence of magneto-electric coupling, which in turn produces a measurable shift in resonant mode frequency as a function of twist angle. We show that this coupling mechanism is equivalent to introducing a non-zero chirality material parameter $\hat{I}{º}_{\text{eff}}$ or axion field $\hat{I}{¸}_{\text{eff}}$ to the medium. Our findings demonstrate the potential for real-time, macroscopic manipulation of electromagnetic helicity.

13 pages, 10 figures