  Optical modes of a multilayered photonic structure with the twisted nematic
liquid crystal as a defect layer have been investigated. The electroconvective
flow in the nematic makes a spatially periodic structure in the form of
abnormal rolls. Non-adiabatic propagation of polarized light in the defect
layer causes unique features of the optical modes corresponding to the ordinary
$o$-waves. The decay of these modes has been demonstrated with increasing
voltage due to the effect of cross-polarization diffraction loss. The modes
short-wave shift resulting from the contribution of the non-adiabatic geometric
phase to the total phase delay of the wave during a round-trip propagation
through the photonic structure has been found both experimentally and
numerically
