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 nonzero electromagnetic helicity as a result of the coupling of near degenerate ${\mathrm{TE}}_{11(p+1)}$ and ${\mathrm{TM}}_{11p}$ modes. This can be interpreted as an emergence of magnetoelectric 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 nonzero chirality material parameter ${\kappa }_{\text{eff}}$ or axion field ${\theta }_{\text{eff}}$ to the medium. Our findings demonstrate the potential for real-time, macroscopic manipulation of electromagnetic helicity.

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• Chirality
• Dark matter
• Magnetoelectric effect
• Optical & microwave phenomena
• Photonics
• Axions
• Microwave techniques