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.
We present the results of the investigations of photo galvanic effect in Methyl Red (MR) doped nematic liquid crystal (LC). The photocurrent appears under the action of the active pulsed irradiation. The photocurrent amplitude depends on the cell thickness, temperature, concentration of MR, cell's lifetime and beam propagating direction. Our investigations showed the appearance of the adsorbed MR molecules layer on the boundary surface. We suppose the spatial charge separation in the adsorbed layer of dye molecules.
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.
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.
We report on the first observation of surface-mediated reorientation effect in LC with fullerene-containing aligning polymer. We found a strong light-induced change of the Friedericksz transition voltage in a dc-field. The Friedericksz transition was measured in a LC cell containing reference and command surfaces. The reference surface, covered with the photoaligning material fluoro-polyvinyl-cinnamate (PVCN-F), was irradiated with polarized UV-light providing strong unidirectional planar alignment. The command surface was covered with a mixture of PVCN-F and fullerene in a 2:1 ratio. This layer was also irradiated with UV-light to obtain unidirectional planar alignment. The 90°-twist cell (thickness ≈ 30 μm) was filled with LC 5CB in the nematic phase. We found a strong increase of the Friedericksz transition voltage as the intensity of the incident beam from He-Ne laser was increased. The effect was reversible and depended on the sign of electric field applied to the cell. We suggest that the increase of Friedericksz transition voltage arises from charge injection into the LC bulk from the fullerene-containing alignment layer. An enrichment of the ion concentration near the surface causes a redistribution of the electric field in the cell, localizing it more strongly near the surface, and, thus, leading to an increase of the voltage necessary to reorient the director.
The effect of a hidden photoalignment of nematic liquid crystal (LC) after the irradiation of the cell with linearly polarized light in the isotropic phase was recently observed (Phys.Rev.E, 021701, 63, (2001). It was found that the effect was caused by light-induced desorption and adsorption of dopant dye molecules on the aligning surface. The desorption of the dye molecules resulted in the anisotropy axis perpendicular to the light polarization whereas the light-induced adsorption causes the anisotropy parallel to the light polarization vector. Competition between these processes determined the resulting direction of the easy axis in a nematic phase. In the present paper we distinguished these two mechanisms by using different geometries of the irradiation of the cell.
We found a thermally-induced threshold reorientation of the nematic liquid crystal from homeotropical alignment to planar one. The fenomenon was observed in a symmetric cell with inner surfaces covered with fluoro polyvinyl-cinnamate. This threshold reorientation is an anchoring transition of the second order. We suggest that a competition of aligning abilities of flexible polymer fragments and main polymer chains is the basic reason for this phenomenon.
We consider the effect of magnetically controlled anchoring of ferro-nematic suspensions. Together with co-authors we recently found that application of weak magnetic field to a cell filled with the ferro-suspension induced an axis of easy orientation of the director of liquid crystal on a polymer surface (Mol. Cryst. Liq. Cryst., 375, 81, (2002). Here we present a simple theoretical model of the effect that assumed coupling between liquid crystal and the applied magnetic field. This coupling is assumed to be a combination of the interaction between magnetic field with ferro-particles magnetic momentum on the one hand and LC director anchoring with these particles on the other hand.
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.