Distinct Berry phases in a single triangular Möbius microwave resonator

We report the experimental observation of two distinct Berry phases ($+\frac{2π}{3}$ and $-\frac{2π}{3}$) generated on the surface of a Möbius cavity resonator at microwave frequencies supporting the TE$_{1,0,n}$ mode family. This resonator consists of a twisted, mirror-asymmetric prism with a cross-section of the triangular $D_3$ symmetry group, bent around on itself to form a ring. This geometric class supports resonant modes with nonzero electromagnetic helicity (i.e. nonzero $\vec{E}\cdot\vec{B}$ product) at microwave frequencies. There exist modes with three-fold rotational symmetry as well as those that exhibit no rotational symmetry. The latter result in an accumulated Berry phase whilst the former do not, which is determined from the measured frequency shift of the modes when compared to a mirror-symmetric resonator of otherwise equivalent geometry.