Point of ultra-sensitivity to perturbations for axial propagation in helicoidal bianisotropic structures

It is shown that under a certain condition the optical properties of helico idal bianisotropic structures for propagation along the helix exhibit drast ic changes. An inhibition of the Bragg reflection occurs and the structure behaves optically as an isotropic medium. A very small perturbation to the structure can then induce a strong optical effect.