Electrodes and alignment layers are major components of practically any liquid crystal device. Typically, indium-tin oxide films serve as electrodes (to apply an external voltage across the liquid crystal layer) whereas polyimide-based polymers (alignment layers) provide the required alignment of liquid crystals. Conducting polymers can combine the afore-mentioned two functions thus serving as both electrodes and alignment layers. In the majority of the reported studies, highly conducting polymers were used. On the contrary, in this paper, we report on electro-optics of liquid crystal cells utilizing weakly conducting polymers. Both static and time electro-optical response are analyzed. The designed cells are characterized by some interesting electro-optical features including the dependence of the effective threshold voltage on the frequency (0-1000 Hz) of the applied electric field. The model of this frequency dependence of the effective threshold voltage is discussed. Our results suggest that weakly conducting polymers are very promising materials for the development of flexible and wearable liquid crystal devices.
In this paper, electro-optical properties of sandwich-like cells utilizing polymers (either weakly conducting or ferroelectric) and nematic liquid crystals are reported. The tuning of the threshold voltage for electrically controlled birefringence is demonstrated. Physical mechanisms of this tuning are discussed.
There are two switching processes where observe in polymer-dispersed liquid crystals (PDLC) when pulse electric field applied: - Slow switching process with rise time hundreds microseconds; - Fast switching process with nanoseconds rise time. The result of research, design and testing ultra-fast PDLC optical gate is presented. The feasibility of 100 nsec rise time optical gate with 1 square inch crystal clear transmission (better than 1.54 dB) and attenuation in OFF state more than 26 dB (30.4 dB for two serial layers) for non-polarized light has been shown.
Liquid crystal alignment is a key process in the production of modern display devices and other liquid crystal based
optical elements. The traditional alignment procedures, dealing with treatment of organic films (rubbing, photopolymerization,
ion beam treatment, etc), do not satisfy the increasing demands of modern technology. Because of the
presence of a degradable polymer layer, none of these methods provides stability of the liquid crystal orientation for
operation in high intensity visible, UV or infrared light. This problem can be solved using anisotropically treated
inorganic thin films. The alignment is very sensitive; both polar and azimuthal anchoring energy parameters can be
controlled by either conditions of the treatment process or the history of the inorganic film formation. The non-organic
nature of the substrates provides non-degrading stability of the liquid crystals alignment while operating in high intensity
visible, UV or infrared light. We demonstrate and discuss the use of this technology in many light modulating devices,
including high intensity light phase retarders and displays operating at harsh ambient conditions.
Ferroelectric nanoparticles significantly improve properties of existing liquid crystals and benefit the performance
of many devices. By changing a concentration and a type of ferroelectric particles one can affect physical properties of
the nematic, smectic, and cholesteric liquid crystal materials, including the dielectric constants, the birefringence, the
phase transition temperatures, and even the order parameter. We demonstrate the performance of these new materials in
various devices, including displays, light modulators, and beam steering devices.
The stressed liquid-crystal (SLC) electro-optic effect promises fast electro-optic response times even for design wavelengths
in the infrared (IR). Here we report characteristics of SLC devices appropriate for use as liquid-crystal-onsilicon
(LCOS) spatial light modulators (SLMs) in the near ( λ = 1.8-2.5 μm), mid (3-5.5 μm) and far (8-14 μm) IR
bands. For these three bands we fabricated SLC devices with 5, 10, and 20 μm thicknesses; at drive voltages of 25, 50,
and 125 V respectively these devices gave half-wave modulation with response speeds in the 1.3-1.6 ms range. Visiblelight
measurements on a 20-μm-thick SLC device between crossed polarizers gave a contrast ratio of 360:1 which
improved to nearly 18,000:1 with a Babinet-Soleil compensator offsetting residual SLC retardance. Widely available
high-voltage options in standard CMOS processes offer sufficient drive for near- and mid-IR SLCOS devices; with
modest increase of SLC material birefringence Δn and dielectric anisotropy Δε far-IR devices would be feasible, too.
Pixel drivers utilizing these options have pitches less than 24 μm, making 1000 ×1000 SLMs feasible.
Stressed liquid crystals (SLCs) have been applied in fields such as optical phase array non-mechanical beam steering applications, adaptive optical tip-tilt correction, and fast displays because SLCs are capable of switching large phase shift in sub-millisecond time ranges. SLCs consist of liquid crystal micro-domains dispersed in a stressed polymer matrix. In this paper, we propose a model of close-packed, shaped liquid crystal droplets inside a sheared polymer matrix based upon the measurements of polarizing microscopy, fluorescence confocal microscopy, and visible-near-infrared spectroscopy. The light scattering of SLC films results mostly from the index mismatch between adjacent liquid crystal domains instead of the index mismatch between polymer matrices and liquid crystals as in traditional polymer dispersed liquid crystals. We show how the light scattering of SLC cells is greatly reduced upon shearing because the liquid crystal domains are aligned along the direction of shearing. The stretching of polymer matrices and the reshaping of liquid crystal domains upon shearing are confirmed by fluorescence confocal microscopy. The calculations of the electro-optic responses are based on the balance between the elastic torque and the electric field torque. Our experimental results support the calculations.
Recently discovered stressed liquid crystals (SLCs) are of a great interest because they provide largest phase retardation shift achievable within shortest time interval. This effect was accomplished by decoupling the speed of a liquid crystal layer from its thickness. SLCs easily switch 5 microns of the phase retardation at sub-millisecond speeds while 50 microns requires only several milliseconds. SLCs are therefore able to modulate the IR light with the frequencies higher than 10 kHz. The SLCs are polymer/liquid crystal composites; however, their electro-optic properties differ significantly from previously developed polymer dispersed liquid crystals and polymer network/stabilized liquid crystals. The applied stressed force aligns the domains, eliminating scattering and hysteresis at the same time. The phase shift is highly linear with the applied voltage, greatly simplifying the drive electronics. The SLCs pose intriguing basic scientific questions and may be used in a lot of new electro-optical applications (micro-displays, diffractive optical elements, beam steering devices).
Since the pioneering works of Yablonovitch and John, the concept of photonic crystals has attracted great attention from both fundamental and practical points of view. Different types of approaches have been taken to realize the spatial periodic structures: nanolithography techniques developed to produce semiconductors, sedimentation of monodispersed nanoscale spheres, or holographic illumination of photosensitive materials. In our work, we employed two newly discovered fascinating phenomena: particle drag effect and particle pumping effect in a liquid crystal to build the ordered colloidal structures. Combining the moving nematic-isotropic transition line with a patterned electric field can be used to move particles from one place to another. This can be used to pack particles in a certain place in an ordered periodic structure. The speed of the interface and the magnitude of the applied electric field controls the size, density and/or dielectric property of the particle that can be moved and determines those that are left behind. This capability allows us to place “defects” at particular locations in the photonic crystals constructed. Although many challenges remain before this system can be used in practical optical components, this new technique provides an excellent means of producing complex photonic crystals tailored for specific optical affects and applications.
The recently discovered stressed liquid crystals (SLCs) are of a great interest because they provide the largest phase retardation shift achievable within the shortest time interval. This was accomplished by decoupling the speed of a liquid crystal layer from its thickness. SLCs easily switch 5 microns of phase retardation at sub-millisecond speeds. We have produced phase shifts as large 50 microns in several milliseconds. SLCs are therefore able to modulate IR light with response frequencies higher than 10 kHz. The SLCs are polymer/liquid crystal composites; however, their electro-optic properties differ significantly from previously developed polymer dispersed liquid crystals and polymer network/stabilized liquid crystals. An applied mechanical stress aligns the liquid crystal domains, eliminating scattering and hysteresis at the same time. The phase shift is highly linear with the applied voltage, greatly simplifying the drive electronics. The SLCs pose intriguing basic scientific questions and may be used in a host of new electro-optical applications (micro-displays, diffractive optical elements, beam steering devices).
Long-range forces between ultra-fine particles imbedded in liquid crystal (LC) result in intriguing colloids. Embedded inorganic particles in LC contribute to the properties of the LC matrix. Large (>>mkm) colloidal particles form defects in LC matrices due to strong director deformations and ensembles of these particles and defects can form complex structures. Small particles at its high concentration (> 2-3% by weight) create almost a rigid LC suspension. We show that at low concentrations LC submicron colloids appear similar to a pure LC with no readily apparent evidence of dissolved particles, but possess unique properties. The diluted suspensions are stable, because the small concentration of submicron particles does not significantly perturb the director field in the LC, and interaction between the particles is weak. At the same time, the submicron particles share their intrinsic properties with the LC matrix due to the anchoring with the LC. We report on the development and unique properties of the diluted suspensions of ferroelectric submicron particles. Our results show that doping a nematic LC matrix with ferroelectric submicron particles results in a suspension, which possesses an enhanced dielectric anisotropy and reveals ferroelectric and paraelectric properties inherent to the submicron particles. In particular, we observed essential decrease of the driving voltage of the quadratic dielectric response and non-usual linear dipole response of the suspensions on the application of ac-field. We present a theoretical model of dielectric properties of ferroelectric suspensions.
The sticking effect on photoaligning surfaces was investigated. We demonstrated that additional irradiation of photoaligning polymers with cinnamoil side groups with not-polarized UV-light strongly decreases their sticking parameter. We associate this effect with cross-linking of the flexible side-groups by UV light and, as a consequence, with light-induced strengthening of the photoaligning surface. Restriction of mobility of the flexible groups on the photoaligning surface (surface strengthening) resulted in depressing the sticking effect. The method of the decrease of the sticking effect by the light-induced strengthening is rather general, and it can be applied for any photoaligning materials undergoing a light-induced cross-linking of polymer fragments. For fluoro polyvinyl-cinnamate the light-induced strengthening allowed us to get the record value of the sticking parameter, S0 = 0.2%, which is better than traditional rubbed polyimide surfaces provide. Such a value of the sticking parameter along with other aligning characteristics allows considering fluoro polyvinyl-cinnamate as very prospective material for modern liquid crystal display technologies.
Long-range forces between ultra-fine particles imbedded in liquid crystal (LC) matrices result in intriguing colloids. Embedded inorganic particles in LC contribute to the properties of the LC matrix. For example, doping of a LC with ferromagnetic particles resulted in a strong enhancement of magnetic properties of the LC. Large (>>µm) colloidal particles form defects in LC matrices due to strong director deformations and ensembles of these particles and defects can form complex structures. Small (<<µm) particles at its high concentration (>2-3% by weight) create almost a rigid LC suspension. Here we show that at low concentrations LC nanocolloids appear similar to a pure LC with no readily apparent evidence of dissolved particles, but possess unique properties. The diluted suspensions are stable, because the small concentration of nanoparticles does not significantly perturb the director field in the LC, and interaction between the particles is weak. At the same time, the nanoparticles share their intrinsic properties with the LC matrix due to the anchoring with the LC. In particular, doping a nematic LC matrix with ferroelectric nanoparticles results in a suspension, which possesses an enhanced dielectric anisotropy and reveals ferroelectric and paraelectric properties inherent to the nanoparticles.
In a matrix of an oligomeric liquid crystal hydrophilic aerosil particles form agglomerates of about 1 (mu) length which consist of aggregates with a diameter of about 150 nm, observed by AFM. In the same matrix, but with hydrophobic aerosil, smaller aggregates with a length-to-breadth ratio 100 nm/60 nm appear. In thicker areas of both samples after heating to clearing temperature and cooling down the cholesteric fingerprint texture occurs, where the lower pitch is observed for the sample containing the hydrophilic aerosil.
The change of the molecular organization in the liquid crystal (5CB)-aerosol system under the influence of electric field is investigated with the use of IR spectroscopy methods. It is shown that aerosol A-300 addition (10 weight %) to 5CB leads to forming weak hydrogen bonds between the aerosol hydroxyl groups and the (pi) -electrons of benzene rings of 5CB (Si-O- H...(pi) ). Under the influence of electric field (U equals 60 V, f equals 2 kHz) these bonds undergo a rearrangement, the 5CB molecules in the aerosol pores align along a direction of the electric field and then form the new network of hydrogen bonds. In the case of the aerosol modified by the polymethylsyloxane the formation of new hydrogen bonds does not take place and in the electric field the 5CB molecules are easily oriented along the field direction.
Dependences of an irreversible response (memory effect) of suspension 'liquid crystal -- aerosol' on the ingredient's properties and electric field parameters are investigated. The experimental results are discussed in the frame of a model of a network organization of aerosol in a nematic liquid crystal.
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