From selective to wide-band light reflection: a simple thermal diffusion in a glassy cholesteric liquid crystal

Due to their helicoidal structure, cholesteric liquid crystals exhibit rema rkable optical properties. Selective light reflection occurs when the pitch (repeat distance) is of the order of the wavelength of incident light prop agating along the helix axis. The wavelength bandwidth, due to the optical anisotropy, is typically limited to 50 nm which is insufficient for some ap plications (full-colors displays, for example). By introducing a pitch grad ient in the helix during a novel two-step process in a cholesteric glass, w e show that reflection may occur over a wavelength bandwidth greater than 3 00 nm. First, the reflection bandwidth is adjusted by thermal annealing. Th en, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient esta blishment and film hardening, are independently controlled. The present pro cess exhibits some reversibility and properties intrinsic to the glassy sta te are gained: laser-writing high resolution full-color images on solid fil ms for image recording or high-density optical data-storage are indeed conc eivable.