Pretransitional chiral LC phase: Optical characteristics and lasing

The pretransitional phase was observed in three component chiral systems, consisted of two nematogenic liquid crystals (LC) with different phase sequences (one with smectic A phase and another with smectic C phase) and optically active nonmesomorphic dopant. In the temperature interval, corresponding to the intermediate phase, between cholesteric and smectic phases an unusually low intensity of transmitted light (~20%) was observed. The investigation of circularly polarized light shows that this structure transmitted both the right and left handed circularly polarized light. We doped to this 3-component mixture the luminescent dye. We investigated the luminescent chractericstics for the case when the wavelength of selective reflection band coincides with luminescence wavelengths in cholesteric and the intermediate phases. The suppression of luminescence was greater in the intermediate phase than in the cholesteric phase. To obtain laser emission in intermediate phase the samples were pumped by nitrogen laser. The lasing was observed at the long wavelength edge of the photonic band gap. Regarding the structure of this intermediate phase an alternative model of twist grain boundary (TGB) phase is proposed. The model represents a layered structure where between cholesteric layers the smectic A layers oriented normally to the cell surfaces, and thus with director parallel to the surfaces are formed.