Polarization-dependent filters based on chiral photonic structures with defects

A one-dimensional photonic device to perform optical multiple-wavelength filtering for light of a particular circular polarization state is presented and modelled. It is shown that the transmission spectrum of a stacked structure consisting of two cholesteric films with the same pitch gradient in the helix can be tailored by incorporating the defects of pitch jump and phase jump in the middle of the structure. The introduction of the defects results in the exhibition of multiple-band-pass characteristics inside the photonic band gap when incident circularly polarized light has the same handedness as the cholesterics. Some critical parameters are analysed: the thickness of the double-composite film, co-polarized/cross-polarized transmittance and reflectance, and the twist angle. For light of specified circular polarization, the shift of the filter in transmitted wavelength can be tuned linearly by adjusting the amount of the phase jump. The proposed chiral filter may be useful for multichannel lightwave systems. Some rules for the practical implementation of a filter with polarization sensitivity are given.