Bragg reflections in cholesteric liquid crystals: From selectivity to broadening and reciprocally

Due to the existence of a macroscopic helicoidal structure, cholesteric liquid crystals exhibit many remarkable properties like selective light reflections. Bragg reflections occur when the helix pitch is of the order of the wavelength of the incident light propagating along the helix axis. The wavelength bandwidth is due to the medium birefringence and is typically limited to 50 nm, which is insufficient for some applications (full-colors displays, for example). The purpose of this paper is to show that, by favoring a pitch gradient in the helix during a novel experimental process, it is possible to get a cholesteric glassy material whose spectral characteristics evolve from a selective to a broad-band filter Light reflection occurs on several hundreds of nanometers and several intermediary optical states are available during a thermal treatment. From a reciprocal viewpoint, the inverse transition, from a broad- to a narrow-band filter, is also demonstrated. Such a process is driven by one parameter: the annealing temperature. Then, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled. Different possible stages occuring during the thermal history of the sample are discussed. The very glassy state of these reflectors would give new opportunities for optical data storage.