Electric Field Effects and Two Frequency Colour Switching in the Cholesteric and Blue Phases of Nematic/Cholesteric Mixtures

Colour switching has been studied in the blue phases of a chiral nematic system as a function of the voltage and frequency of an applied electric field. In the BPI two effects were observed. Above a critical frequency, small wavelength shifts (∼ 50nm) were observed whilst, below this frequency, a cholesteric fan structure was induced. This fan texture was stored on removal of the field and reapplication of an a.c. signal induced a change in the reflected colour from blue to red. In the BPII, a voltage dependent critical frequency was also observed. Above this frequency, the selectively reflected colour could be switched reversibly from orange to blue or green in a time faster than 40 ms. Below the critical frequency, a turbulent scattering texture was induced and this provided an erasure mechanism for the colour switch.