In-situ formation of polymer-stabilized/-free cholesteric bi-layer photonic crystal

Tunable photonic crystals based on cholesteric liquid crystals (CLCs) have attracted considerable attention due to their tunable optical properties and self-assembly capabilities. Despite the rapid development of various CLC-based devices, the narrow bandwidth of the photonic bandgap in CLCs limits their use in some practical applications. This paper presents a method for forming polymer-stabilized and polymer-free cholesteric bilayer photonic crystals in polymer-stabilized cholesteric liquid crystals (PSCLCs). By tuning the concentration of reactive monomers and controlling the UV polymerization conditions, two different PBGs can be formed within a single PSCLC cell. As the concentration of RM257 increases to 40 % and the intensity of UV light is used at 22 mW/cm², the distance between the two reflectance bands can reach 200 nm, reflecting green and red light. The dynamic formation process and the tunability of these cholesteric bilayer structures under electric fields and temperature variations are investigated. By controlling the temperature and electric field, the reflectance of the liquid crystal layer can be reduced from 50 % to 15 %, resulting in a single-band reflection. In addition, we have proposed a tunable polarization volume grating based on the cholesteric bi-layer photonic crystal, which can diffract two specific light colors.