In this paper, we report a computational and experimental study using tunable infrared (IR) metamaterial absorbers (MMAs) to demonstrate frequency tunable (7%) and amplitude modulation (61%) designs. The dynamic tuning of each structure was achieved through the addition of an active material—liquid crystals (LC) or vanadium dioxide (VO<sub>2</sub>)--within the unit cell of the MMA architecture. In both systems, an applied stimulus (electric field or temperature) induced a dielectric change in the active material and subsequent variation in the absorption and reflection properties of the MMA in the mid- to long-wavelength region of the IR (MWIR and LWIR, respectively). These changes were observed to be reversible for both systems and dynamic in the LC-based structure.
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.
Unique liquid crystal (LC) spatial light modulators (SLM) are being developed for foveated imaging systems that provide wide field-of-view (FOV) coverage (±60° in azimuth and elevation) without requiring gimbals or other mechanical scanners. Recently, a transmissive-SLM- based system operating in the visible (532 nm) has been demonstrated. The LC SLM development is addressing implementation issues through the development of high figure-of-merit (FoM) LC materials and transmissive high-resolution SLMs. Transmissive SLM operation allows the foveated imaging configuration to be very compact using a very simple lens system. The reduction in the size, weight and cost of the imaging optics and in data acquisition/processing hardware makes the foveated approach attractive for small platforms such as unmanned airborne vehicles (UAVs) or missile seekers.
Low viscosity high birefringence nematic liquid crystalline compounds and mixtures are described. They belong to the families of 4'-alkyl-3-fluoro-4-isothiocyanatobiphenyls, 4"-alkyl-3,5-difluoro-4-isothiocyanatoterphenyls, 4'-alkyl-3-fluoro-4-isothiocyanatotolanes and 4'-(4-alkylphenyl)-3,5-difluoro-4-isothiocyanatotolanes.
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.
Most liquid crystal display (LCD) devices use two ITO-glass substrates in order to confine the fluidic LC. To align the LC molecules, the inner surfaces of the substrates were coated with a thin polyimide (PI) layer. These PI layers are mechanically buffed in order to produce uniform molecular alignment. To reduce weight, the single-substrate approach has been explored recently in which the LC device consists of a substrate and a thin polymer film. The major technical challenge is how to align the LC molecules on the polymer film side. In this paper, we demonstrate a new single-substrate IPS-LCD. The LC cell consists of an anisotropic LC polymer film and an interdigitated ITO-glass substrate. The anisotropic film not only behaves as a substrate but also helps align the LC molecules. Compared to the LCD using two glass substrates, the new device has almost the same bright state and the same dark state. Our new device exhibits a higher contrast ratio (~514:1) because of good LC alignment. The driving voltage is low, and the response time is reasonably fast. The measured rise time is ~8ms and decay time is ~63 ms using a 12-μm cell gap and E7 LC mixture. This technology is particularly attractive for making single-substrate displays and also has potential for a double-layered guest-host display and a flexible display using IPS LCDs.
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/cm<sup>2</sup>. Two commercial TFT-grade LC mixtures (low birefringence MLC-9200-000 and high birefringence TL-216) exhibit a longer lifetime in SiO<sub>2</sub> 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 SiO<sub>2</sub> layers, high optical density UV filter, longer cutoff-wavelength blue filter, and low birefringence LC materials should be considered.
We report the synthesis procedures, phase transition temperatures, optical and dielectric properties of the fluoro and chloro -- substituted two rings and three rings isothiocyanatotolanes. Three eutectic mixtures comprising of these isothiocyanatotolane compounds showing a wide nematic range, low viscosity and high birefringence (~0.4) are formulated.
Several linearly conjugated isothiocyanato (NCS) tolanes, biphenyl-tolanes and bistolanes were synthesized and their properties evaluated. Eutectic mixtures compromising these compounds were formulated. The temperature dependent birefringence, visco-elastic coefficient and figure-of-merit were measured. These isothiocyanato tolane mixtures exhibit a favorably high figure of merit at elevated temperatures. Potential applications of these mixtures for laser beam steering, tunable-focus lens and telecom variable optical attenuator are emphasized.
Isothiocyanato tolane liquid crystals exhibit a high birefringence and relatively low viscosity. The phase transition temperatures, birefringence and visco-elastic coefficient of several high birefringence isothiocyanato tolane compounds are evaluated. Using these compounds, several eutectic mixtures were formulated. These new mixtures exhibit a wide nematic range, high birefringence (>0.4) and low viscosity.
In some mixtures composed of ferroelectric components the induced antiferroelectric phase was observed. Two mixtures of this kind, both exhibiting antiferroelectric order in a broad concentration range, were investigated. The effect of electric field of various strengths on optical properties of these mixtures was studied. In small electric fields the modulation of light intensity occurred, which amplitude in ferroelectric phase was much higher than in the antiferroelectric one. In mixtures with induced antiferroelectric phase we detected a large hysteresis of the transition temperature between ferroelectric and antiferroelectric sates. In high electric fields thresholdless switching was observed in the hysteresis range. The presence of the thermal hysteresis of the C<SUB>A</SUB><SUP>*</SUP> - C<SUP>*</SUP> phase transition can be considered as a mark of the V-shaped switching. In bulk samples the C<SUP>*</SUP>- phase is stable in, this temperature region, where the V- shaped switching was observed in thin samples.
The aim of our work was the investigating of compounds not having SmC<SUB>A</SUB>* phase, but in their mixtures for some concentration ranges induced smectic SmC<SUB>A</SUB>* was observed. Physical macroscopic properties such as temperature dependencies of the spontaneous polarization P<SUB>S</SUB>, the tilt angle (theta) and the electric permittivity (epsilon) <SUB>(perpendicular</SUB>) were measured for three compounds and their mixture. The investigated mixture exhibited antiferroelectric order in a broad concentration range. The large thermal hysteresis of the transition between ferroelectric and antiferroelectric phases for this mixture was demonstrated using dielectric method. We have tried to explain the origin of the induced SmC<SUB>A</SUB>* phase on the basis of the molecular structure of molecules of investigated compounds obtained from theoretical study by using semi-empirical quantum mechanical calculations method MINDO/3.
Chiral nematic polar mixtures (7BCB-7CN)-5<SUP>*</SUP>CB were investigated by visual and spectrophotometric methods. It was found that these systems have a reverse temperature dependence of selective light reflection. The wavelength of reflected light increased with the temperature increase. The bicomponent mixture containing 5<SUP>*</SUP>CB and non-polar 805, showing enhanced A<SUB>1</SUB> phase, had a normal spectral characteristics. The color of reflected light changed from red to blue with temperature increase in the neighborhood of S<SUB>A1</SUB>-N<SUP>*</SUP> phase transition.