External cavity-free and electrically tunable laser made from photonics band gap (PBG) materials with electrically tunable stop band is reported. The tunable PBG materials are developed from a family of novel cholesteric liquid crystals (CLC) with electrically variable pitch that adopts a non-constant distribution in space across the CLC film. The CLC exhibits a distributed feedback cavity whose resonant frequency can be electrically varied over a spectral range wider than 300 nm. Under an optical pumping and subject to a variable electric field, a tunable laser has been demonstrated in experiment that shows a wavelength tuning over 33 nm.

Multiple phase transitions and monotropic phase behavior are observed in a discotic liquid crystal, 2,3,6,7,10,11-hexa(4‘-octyloxybenzoyloxy)triphenylene (HOBT-C8). Detailed phase structural analyses using wide-angle X-ray diffraction and electron diffraction show that, in addition to a discotic nematic (ND) phase at high temperatures, a rectangular columnar (ΦR) phase and an orthorhombic crystalline (KI) phase developing sequentially upon cooling can be identified. A crystalline (KII) phase with a higher melting temperature than that of the KI phase forms during heating. Therefore, the KI phase represents a metastable phase with respect to the KII phase, and the more stable KII phase can be bypassed during cooling at relatively fast cooling rates. The KI phase is thus monotropic with respect to the KII phase at relatively slow heating rates. The optical texture of each phase can be distinguished from polarized light microscopy. A relationship between Gibbs free energy and temperature for different phases can be constructed, which thermodynamically and kinetically represents these multiple phase transitions and the monotropic phase behavior of the KI phase.

Ultraviolet (UV) polymerizable discotic liquid‐crystalline (DLC) molecules (2,3,6,7,10,11‐hexakis(4′‐acryloy‐m‐alkyloxybenzoyoxy)triphenylene [HAHBT‐m, where m was the number of methylene units, and here m = 6 (HAHBT‐6)]) were assembled to form a negative retardation film with an oblique optical axis on a specifically designed rubbing‐aligned polyimide layer surface [6FDA‐11CBBP (where 11 is the number of methylene units in the side chains)]. The side chains of this polyimide were terminated by cyanobiphenyl groups. Surface‐enhanced Raman scattering (SERS) and optical second harmonic generation results showed that rubbing caused a surface structural re‐arrangement in the alignment layer resulting in a negative pre‐tilt angle (θs) of –8.5° (which was in the direction opposite to the rubbing direction). The molecular topology at the rubbed surface was governed by a stable fold‐like bent structure of the cyanobiphenyl side chains, in which the CN groups preferentially pointed down towards the surface. When the DLC molecules were deposited onto the alignment surface and polymerized via UV irradiation to generate a new optical film, an oblique optical axis with an average tilt angle of –18.6° with respect to the film normal was detected using ellipsometric measurements. This tilted optical axis was developed by the DLC molecules being wedged on top of the cyanobiphenyl groups when in the bent conformation. Furthermore, the tilt angle difference between the θs at the alignment surface and at the air interface of the DLC molecules was attributed to a splay deformation of the DLC molecules along the film surface normal. Optical modeling has also confirmed our experimental observations.

Phase transition behaviors of a discotic liquid crystal 2,3,6,7,10,11-hexa(4‘-octyloxybenzoyloxy)-triphenylene (HOBT-C8) were investigated. Four phases having different orders and symmetries existed in this system:  a discotic nematic phase, a rectangular columnar (ColR) phase, an orthorhombic crystalline (CrI) phase developed upon cooling, and a monoclinic crystalline (CrII) phase formed during heating (Tang, B. Y., et al. Mater. Chem. 2001, 13, 78). The CrI phase represented a metastable phase compared to the CrII phase. On the basis of the results of differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy (PLM) experiments, three crystallization-temperature (T c) regions were identified to help study the crystallization kinetics of the CrI and CrII phases. In the large T c region below 114 °C (region III), the CrI phase formed first despite its metastable nature (monotropic) due to the faster crystallization kinetics of the ColR → CrI phase in this region. The CrI → CrII phase transformation then followed. However, when the T c ≥ 115 °C in region I, which was close to the melting temperature (T m) of the CrI phase (T m = 121 °C), only the CrII phase formed directly from the ColR phase. In the narrow T c region II where 114 °C ≤ T c < 115 °C, primary nucleation and overall crystallization rates of CrI and CrII phases were reversed. Linear crystal growth rate measurements in PLM revealed that a minimum in the CrII phase growth rate was found in region II. This indicated that as long as the growth rates of both phases were similar and on the same growth front, the CrII phase formation could be hampered by the growth of the metastable CrI phase.