  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.
