Preparation of Tunable Photonic Crystal Based on Cholesteric Liquid Crystal

Via introducing cholesteric liquid crystal into voids of colloidal crystal, a novel tunable photonic colloidal crystal based on coupling of cholesteric liquid crystal (CLC) and colloidal crystal (CC) was prepared. Cholesteric liquid crystal used in this paper was prepared by combining nematic liquid crystal 5CB and the chiral dopant COC (cholesteryl oleyl carbonate), its selective reflection wavelength can be controlled by changing proportion of the mixture, and with temperature rising, blue shift of the selective reflection peak occurred. Firstly, SiO2 colloidal crystal was obtained by deposition method, then cholesteric liquid crystals was filled in voids of colloidal crystal by capillary force with temperature above the clearing point. Under polarized light microscope, the cholesteric liquid crystals in voids of colloidal crystal exhibited typical texture of chiral smectic phase in certain temperature region. Since cholesteric liquid crystal has particular selective reflection, if band gap of colloidal crystal locates in the reflection wavelength region of cholesteric liquid crystal, homodromous shift of band gap of cholesteric liquid crystal and colloidal crystal originating from changing of refraction index of cholesteric liquid crystal in voids occurred at the same time with temperature changed. But shift rate of band gap of cholesteric liquid crystal was faster than that of colloid crystal. So when temperature was carefully raised from lower limit of working range of cholesteric liquid crystal, band gap of CLC would gradually approach reflection peak of colloid crystal. Under a certain temperature, band gap of colloidal crystal would couple with selective reflection of CLC and form a single peak, reflection intensity increased from an average of 50% to 74%. With the further increase of temperature, peak of selective reflection of CLC exceeded stop-band of colloid crystal, the single reflection peak re-splitted into double band. Then reversible transition between single and double stop-band of photonic crystal device was realized.