  A slab made of a dielectric structurally chiral medium (DSCM) strongly
reflects the co-handed circularly polarized plane wave, but not the
cross-handed circularly polarized plane wave, in a spectral regime called the
circular Bragg regime. The effect of inserting a central phase defect in a DSCM
slab with a modest number of structural periods is a spectral reflection hole
in the circular Bragg regime, for co-handed incidence only. However, if the
incident plane wave is left-circularly polarized, the geometric phase of the
transmitted plane wave contains evidence of both the circular Bragg regime and
the spectral reflection hole, regardless of the structural handedness of the
DSCM. This evidence is indicative of the type of phase defect. The effect of
inserting a central phase defect in a DSCM slab with a large number of
structural periods is a spectral transmission hole in the circular Bragg
regime, for cross-handed incidence only. The spectral transmission hole may be
difficult to observe experimentally because of absorption inside the DSCM slab,
but it will still be evident in the geometric phase of the transmitted plane
wave, if the incident plane wave is left-circularly polarized.
