Optical phase shifts and diabolic topology in Möbius-type strips

We compute the optical phase shifts between the left and the right-circularly polarized light after it traverses non-planar cyclic paths described by the boundary curves of closed twisted strips. The evolution of the electric field along the curved path of a light ray is described by the Fermi-Walker transport law which is mapped to a Schrödinger equation. The effective quantum Hamiltonian of the system has eigenvalues equal to 0,±κ, where κ is the local curvature of the path. The inflexion points of the twisted strips correspond to the vanishing of the curvature and manifest themselves as the diabolic crossings of the quantum Hamiltonian. For the Möbius loops, the critical width where the diabolic geometry resides also corresponds to the characteristic width where the optical phase shift is minimal. In our detailed study of various twisted strips, this intriguing property singles out the M”obius geometry.