Photonic defect modes of cholesteric liquid crystal with spatially varying pitch

The introduction of photonic defect modes in a cholesteric liquid crystal (ChLC) can be achieved by a global deformation of the helix with a chiral anti-symmetry relative to the middle of the ChLC configuration. The defect modes can be excited only when the circularly polarized incident wave has the same handedness as the ChLC. The tuning of the defect modes can be realized by adjusting the pitch gradient. Whereas, the transmittance of the defect modes is greatly reduced when the structure thickness is large far away from a critical value, and even some defect modes will be completely suppressed. A forbidden band for polarization-selective reflection can be obtained, and the bandwidth is much wider than that in a monotonically varying pitch structure.