Polarization properties of cubic blue phases of a cholesteric liquid crystal

In this paper, we have experimentally investigated polarization properties of the blue phase of a cholesteric liquid crystal in polycrystalline cells with different alignment layers. Experiments were carried out by various temperatures and wavelengths within the spectral range covering Bragg reflection. It was conclusively demonstrated that the change of polarization state of transmitted light through polycrystalline BP-cell is mainly due to Bragg reflection, while optical activity is relatively small. Besides, the linear birefringence was not observed, as expected. It was shown that the ellipticity of the outgoing polarization state can change from almost linear (1.33%) to elliptic (7.33%), depending on sample orientation, with negligible changing of optical rotatory power (for BP I at 470 nm). The results indicate that also polycrystalline BPLC structure, although locally anisotropic, is macroscopically isotropic showing non-negligible optical activity only for resonant wavelengths, although being much lower than that in a typical cholesteric phase. It was also shown that different alignment layers in BP-cell may shift Bragg reflection spectral range, so influencing the outgoing polarization state for particular wavelength.