Physical properties and alignment performance of biphenyl and terphenyl negative dielectric anisotropic liquid crystal (LC) compounds are investigated. Results show biphenyl compounds align well in homeotropic LC cells and the alignment of terphenyls are relatively poor. We have developed a new method to align these high birefringence LC compounds. Adding a few percent of positive dielectric anisotropic or nonpolar LC material not only enhances the contrast ratio but also improves the overall figure-of-merit. Molecular modeling and experimentation are demonstrated to support this concept.
We have developed a non-contact birefringence probing method for studying the dielectric heating-induced temperature rise of dual-frequency liquid crystals (DFLCs). The dielectric heating effects of three DFLC mixtures are investigated quantitatively. By properly choosing the molecular structures, the dielectric heating effect can be minimized while keeping other desirable physical properties uncompromised.
High birefringence liquid crystals (LCs) play an important role for laser beam steering, tunable-focus lens, reflective display, cholesteric LC laser, infrared dynamic scene projector, and telecom variable optical attenuator applications. We have developed some high birefringence compounds and eutectic mixtures with birefringence in the 0.4-0.7 range. For some photonic devices where response time is critical, we have also developed high birefringence dual-frequency LC mixtures. The cross-over frequency is around 5-10 kHz. Using such a dual-frequency LC mixture, sub-millisecond response time is achieved.
The UV stability of empty liquid crystal (LC) cells incorporating commercial polyimide (PI) and silicon-dioxide (SiO2) alignment layers under nitrogen environment and the vacuum-filled LC cells were studied. Experimental results show that the molecular alignment of PI cells is degraded after 10 hours of UV (l~365 nm) illumination at intensity I~350 mW/cm2. Two commercial TFT-grade LC mixtures (low birefringence MLC-9200-000 and high birefringence TL-216) exhibit a longer lifetime in SiO2 cells than in PI cells. Moreover, MLC-9200-000 has a much longer lifetime than TL-216. To lengthen the lifetime of a LCD projector, UV transparent PI layers or inorganic SiO2 layers, high optical density UV filter, longer cutoff-wavelength blue filter, and low birefringence LC materials should be considered.