Narrowband and ultranarrowband filters with electro-optic structurally   chiral materials
  When a circularly polarized plane wave is normally incident on a slab of a structurally chiral material with local $\bar{4}2m$ point group symmetry and a central twist defect, the slab can function as either a narrowband reflection hole filter for co-handed plane waves or an ultranarrowband transmission hole filter for cross-handed plane waves, depending on its thickness and the magnitude of the applied dc electric field. Exploitation of the Pockels effect significantly reduces the thickness of the slab. 
Akhlesh Lakhtakia