Electrically Controlled Bandgap in a Chiral Material

The propagation of light along the axis of nonhomogeneity of a structurally chiral medium (SCM) under the influence of a low-- frequency (dc) electric field aligned along the same axis is investigated. The Pockels effect is assumed to occur, and the SCM is taken to possess locally a $\bar{4}2m$ point group symmetry. The frequency--domain Maxwell curl equations are cast in a 4$\times $4 matrix representation, and the Oseen transformation is invoked. The band structure is analyzed, as are also the eigenvectors as well as the transfer matrix. Finally, the reflection and transmission of a plane wave due to a SCM of finite thickness is considered. The low--frequency electric field is shown to control the bandgap.