Quantitative conoscopic imaging of cholesteric thin films

The periodic helical structure of an aligned polymerized cholesteric liquid crystal (ChLC) film gives rise to circular Bragg reflection within a narrow wavelength range, such that circularly polarized light of the same handedness as the helix is reflected, while counter circularly polarized light is transmitted. The ChLC is also strongly optically active with opposite rotatory power at either side of the circular Bragg zone. Both the selective reflection of circularly polarized light within the Bragg zone, as well as the optical activity of the cholesteric liquid crystal, are imaged with a new conoscopic technique based on a microscope equipped with the rotating polarizer-circular analyser MetriPol imaging system. The conoscopic images reveal the off-axis Bragg zone as an annulus at wavelengths below normal selective reflection. The annulus converges into a disk before disappearing at wavelengths above the region for normal selective reflection. The technique allows clear identification of the Bragg zone and the sign of the optical activity.