Dual-frequency Nematic Liquid Crystal Research Articles

Fabrication of high-transmittance and fast-response bistable dual-frequency CLC cells

Liquid crystal devices are popularly used in the design of smart windows. However, the continuous electric field required for their operation may cause unnecessary energy loss and there is a risk of state transition when the electric field disappears. To overcome these problems, bistable devices were developed that exhibit electro-optic memory. In this study, we demonstrate the synthesis of a novel liquid crystalline acrylic chiral dopant. The polymerizable chiral dopant is expected to disperse and be fixed in liquid crystals, homogeneously revealing higher chiral efficiency. Incorporation of the synthesized chiral dopant into the dual frequency cholesteric liquid crystal results in a bistable display performance with a high transmittance of 97.6% and a fast response time of 1.5 ms. Each of the transparent “ON” and opaque “OFF” states was maintained without the bias of a continuous electric field. The achieved results show the possible application of the synthesized acrylic chiral dopant on a bistable dual frequency cholesteric liquid crystal display (BDFCLC) for smart windows and other devices. BackgroundThe continuous electric field required for their operation may cause unnecessary energy loss and there is a risk of state transition when the electric field disappears. MethodsTo fabricate a smart optical device with fast transition between “ON” and “OFF” states, and with continuously electric field addition, a new acrylic chiral dopant was synthesized and mixed with commercially available dual frequency nematic liquid crystal as a new cholesteric liquid crystal material possessing both perpendicular and planer arrangements with and without one-time electric field addition. Significant findingsIn this study, we demonstrate the synthesis of a novel liquid crystalline acrylic chiral dopant. The polymerizable chiral dopant is expected to disperse and be fixed in liquid crystals, homogeneously revealing higher chiral efficiency. Incorporation of the synthesized chiral dopant into the dual frequency cholesteric liquid crystal results in a bistable display performance with a high transmittance of 97.6% and a fast response time of 1.5 ms. Each of the transparent “ON” and opaque “OFF” states was maintained without the bias of a continuous electric field.

Electrically switchable structural patterns and diffractions in a dual frequency nematic liquid crystal

Electrically driven structural patterns in liquid crystals (LCs) have attracted considerable attention due to their electro-optical applications. Here, we disclose various appealing reconfigurable LC microstructures in a dual frequency nematic LC (DFNLC) owing to the electroconvection-induced distortion of the LC director, including one-dimensional rolls, chevron pattern, two-dimensional grids, and unstable chaos. These patterns can be switched among each other, and the lattice constants are modulated by tuning the amplitude and frequency of the applied electric field. The electrically switchable self-assembled microstructures and their beam steering capabilities thus provide a feasible way to tune the functions of DFNLC-based optical devices.

Optical nonlinearity of a dual-frequency nematic liquid crystal via temperature-mediated mapping of dielectric anisotropy.

Liquid crystals are of great interest in the field of nonlinear optics due to their efficient response to low intensity light fields. Here we present a new, to the best of our knowledge, mechanism of a nonlinear optical response which is observed for a dye-doped dual-frequency nematic liquid crystal. The local increase in temperature caused by the absorption of light beam in the liquid crystal medium leads to spatial variation and inversion of the sign of the dielectric anisotropy. When an alternating current electric field with a frequency close to the cross-over frequency is applied to the liquid crystal cell, the planar orientation sustains at the beam periphery, but elastic deformation occurs in the irradiation region. In the case of a dye dopant with negative absorption dichroism, a first-order orientational transition with large bistability region is obtained.

Bistability of Dielectrically Anisotropic Nematic Crystals and the Adaptation of Endothelial Collectives to Stress Fields.

Endothelial monolayers physiologically adapt to flow and flow‐induced wall shear stress, attaining ordered configurations in which elongation, orientation, and polarization are coherently organized over many cells. Here, with the flow direction unchanged, a peculiar bi‐stable (along the flow direction or perpendicular to it) cell alignment is observed, emerging as a function of the flow intensity alone, while cell polarization is purely instructed by flow directionality. Driven by the experimental findings, the parallelism between endothelia is delineated under a flow field and the transition of dual‐frequency nematic liquid crystals under an external oscillatory electric field. The resulting physical model reproduces the two stable configurations and the energy landscape of the corresponding system transitions. In addition, it reveals the existence of a disordered, metastable state emerging upon system perturbation. This intermediate state, experimentally demonstrated in endothelial monolayers, is shown to expose the cellular system to a weakening of cell‐to‐cell junctions to the detriment of the monolayer integrity. The flow‐adaptation of monolayers composed of healthy and senescent endothelia is successfully predicted by the model with adjustable nematic parameters. These results may help to understand the maladaptive response of in vivo endothelial tissues to disturbed hemodynamics and the progressive functional decay of senescent endothelia.

Synchronously controlled optical modes in the transmittance and reflectance spectra of multilayer photonic structure with dual-frequency nematic liquid crystal.

A method for electrically controlled synchronous mode tuning in the transmittance and reflectance spectra of a photonic structure consisting of an asymmetric dielectric Fabry-Pérot microcavity and an ultrathin metallic film has been proposed. The excitation of a broadband Tamm plasmon polariton at a metal/Bragg mirror interface is accompanied by the two phenomena opposing one another. Due to the strong absorption induced by the metal, a sufficient rejection level in the reflection of the off-resonant radiation has been obtained in a wide spectral range, which almost coincides with the photonic band gap, while at the cavity mode frequencies the structure becomes transparent for the reflected radiation. This leads to the appearance of a series of narrow resonance peaks in the reflectance spectra that coincide in frequency with the transmittance peaks. Based on the structural transformations in the dual-frequency nematic liquid crystal used as a photonic structure defect, the tuning of the modes corresponding to the extraordinary waves to both the short- and long-wavelength spectral ranges has been implemented.

Dual-frequency electrically driven nematic microstructures confined to biaxial porous polymer membranes

We report the study of internal ordering and electro-optical response of dual-frequency nematic liquid crystals (DFNLCs) confined to microporous polyethylene terephthalate (PET) membranes. The DFNLCs are characterized by positive/negative signs of dielectric anisotropy in low/high-frequency electric fields E. Microscopic observations assisted by numerical simulations identified the escaped radial (ER) configurations of the nematic director field within the porous membrane for E = 0. Low/high-frequency electric fields were applied to the PET-DFNLC membrane to manipulate the internal nematic configuration. We found that the low/high-frequency electric fields transform the ER structure to the quasi axial/planar radial, resulting in the suppressed/increased propagation of near-infrared electromagnetic radiation through the composite material. The results are discussed for photonic applications.

Dielectric dispersion in a dual-frequency smectic A liquid crystal PPEP(3F6)-CN/P(CN)P6-6 mixture

This study examined the effects of temperature on the dielectric dispersion of cells holding a PPEP(3F6)-CN/P(CN)P6-6 mixture in the smectic A phase with homeotropic or homogeneous alignments, employing frequencies from 100 Hz to 1 MHz. The Cole–Cole function in conjunction with the DC conductivity was employed to assess these dispersion. In the mixture, it was observed that the dielectric anisotropy changed its sign depending on the frequency. This is the first observation of such a dual-frequency characteristic in the smectic phase to the best of our knowledge, although some dual-frequency nematic liquid crystals have been reported thus far. The value of the crossover frequency, where the sign of the anisotropy was observed to change, was determined to be correlated with the relaxation frequency, for the specimen.

Fast Switching Dual-Frequency Nematic Liquid Crystal Tunable Filters.

We develop tunable optical filters with dual-frequency nematic liquid crystal optical retarders to enable fast switching between the passed wavelengths. The filters are composed of a series of two liquid crystal optical retarders. We select the specific thicknesses of the liquid crystal retarders and use individual biasing schemes to continuously tune the wavelength and bandwidth of the filter. This enables fine-tuned filter switching speeds of filter operation in the ms regime. We present theoretical predictions and experimental results for the electro-optical filter characterization as well as an example application for our filter in total internal reflection fluorescence microscopy. We find that our filter switching speeds can be as short as a few ms, an order of magnitude improvement over typical mechanical filter wheel switching speeds. The quality of our fluorescence images is similar to those obtained by conventional filters.

Freedericksz transition in a dual-frequency nematic liquid crystal

The Freedericksz transition was investigated in a dual-frequency liquid crystal consisting of a mixture of 2-chloro-4-(4-pentylbenzoyloxy)benzoate and N-(4-methoxybenzylidene)-4-butylaniline, in which dielectric anisotropy strongly depends on the frequency of the applied ac electric field. Because the threshold voltage for the transition from an undistorted state of orientation to a distorted one is a function of the dielectric anisotropy and of the Frank elastic constants, the threshold voltage was expected to depend on the frequency. The threshold voltage increased steeply as the frequency approached the crossover frequency at which the dielectric anisotropy vanished and the threshold voltage eventually diverged. Furthermore, we examined the Freedericksz transition under ac voltages of two different frequencies.

Order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses

ABSTRACT Basic physical properties of liquid crystals strongly depend on the orientational order parameter. Given a fundamental importance of this parameter, it was measured for a variety of liquid crystals by means of various experimental techniques. At the same time, the reported results were limited to a relatively narrow range of thicknesses of the studied liquid crystal cells (typically, within a 1–20 µm interval). This paper presents the values of the orientational order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses (from 5 to 200 µm). The non-monotonous dependence of the order parameter S on the cell thickness obtained for relatively thin layers (< 60 µm) of dual-frequency nematics is followed by a rapid monotonous decay of S for thicker (60–200 µm) liquid crystal samples. The obtained results are important for the development of emerging, non-display, applications of liquid crystals utilising thick cells and fast switching materials such as dual-frequency nematics. In particular, they provide a basis for understanding recently discovered two regimes of electro-optical switching of thin and thick layers of dual-frequency nematics (Crystals, 9(6), pp. 314, 2019).

Electro-Optical Switching of Dual-Frequency Nematic Liquid Crystals: Regimes of Thin and Thick Cells

Conventional display applications of liquid crystals utilize thin layers of mesogenic materials, typically less than 10 µm. However, emerging non-display applications will require thicker, i.e., greater than 100 µm, layers of liquid crystals. Although electro-optical performance of relatively thin liquid crystal cells is well-documented, little is known about the properties of thicker liquid crystal layers. In this paper, the electro-optical response of dual-frequency nematic liquid crystals is studied using a broad range (2–200 µm) of the cell thickness. Two regimes of electro-optical switching of dual-frequency nematics are observed and analyzed.

Nematic Liquid Crystal Composite Materials for DC and RF Switching

Liquid Crystals (LCs) are widely used in display devices, electro-optic modulators, and optical switches. A field-induced electrical conductivity modulation in pure liquid crystals is very low which makes it less preferable for direct current (DC) and radio-frequency (RF) switching applications. According to the literature, a conductivity enhancement is possible by nanoparticle doping. Considering this aspect, we reviewed published works focused on an electric field-induced conductivity modulation in carbon nanotube-doped liquid crystal composites (LC-CNT composites). A two to four order of magnitude switching in electrical conductivity is observed by several groups. Both in-plane and out-of-plane device configurations are used. In plane configurations are preferable for micro-device fabrication. In this review article, we discussed published works reporting the elastic and molecular interaction of a carbon nanotube (CNT) with LC molecules, temperature and CNT concentration effects on electrical conductivity, local heating, and phase transition behavior during switching. Reversibility and switching speed are the two most important performance parameters of a switching device. It was found that dual frequency nematic liquid crystals (DFNLC) show a faster switching with a good reversibility, but the switching ratio is only two order of magnitudes. A better way to ensure reversibility with a large switching magnitude is to use two pairs of in-plane electrodes in a cross configuration. For completeness and comparison purposes, we briefly reviewed other nanoparticle- (i.e., Au and Ag) doped LC composite’s conductivity behavior as well. Finally, based on the reported works reviewed in this article on field induced conductivity modulation, we proposed a novel idea of RF switching by LC composite materials. To support the idea, we simulated an LC composite-based RF device considering a simple analytical model. Our RF analysis suggests that a device made with an LC-CNT composite could show an acceptable performance. Several technological challenges needed to be addressed for a physical realization and are also discussed briefly.

Investigations of dual-frequency nematic liquid crystals doped with dichroic dye

ABSTRACTIn this work, we studied and analysed a particular variety of liquid crystals, the so-called dual-frequency nematic liquid crystals (DFNLCs). The interest was to perform dielectric spectroscopy and optical characterisation with two pure and doped mixtures of DFNLCs. By means the dielectric spectroscopy we observed a low value of crossover frequency at room temperature for one dye doped mixture. Additionally, in the spectra studies, a scattering of light at cross-frequency, and a stable and unstable focal conical state was observed, respectively, for the investigated mixtures.

Design of Switchable Guided-Mode Resonant Filters in Zenithal-Bistable Liquid-Crystal Gratings

An electro-optically switchable infrared filter based on guided-mode resonances in a zenithal-bistable liquid-crystal grating is designed and theoretically investigated. By proper selection of the material and geometrical parameters, it is shown that the grating can switch between transparent and narrowband filtering operation. Switching times of few milliseconds are predicted owing to the employment of a dual-frequency nematic liquid crystal as the addressable material. The tunability of the resonant wavelength is also studied following two approaches, namely, the adjustment of the angle of incidence of the impinging lightwave and the liquid-crystal tuning by the electro-optic effect.

Nematic liquid crystal mixtures for 3D active glasses application

ABSTRACTMethods of 3D imaging present on the market can be divided into two basic groups: autostereoscopic and stereoscopic. At the stereoscopic methods of 3D imaging it is necessary to use special glasses for separation of images constituting stereo-pair. The most commonly used active glasses consist of two electro-optical transducers based on twisted nematic or other electro-optical effects. Here the evaluation of liquid crystal mixtures is designed intentionally for 3D active glasses in order to eliminate drawbacks of active glasses available on the contemporary market. Material parameters of new designed liquid crystal mixtures such as: elastic constants, viscosity, refractive indices, dielectric and spectral properties as well as response times have been determined. In addition to the symmetrisation of the switching times of 3D active glasses, a dual frequency nematic liquid crystal has been proposed.

Switchable Fresnel lens based on hybrid photo-aligned dual frequency nematic liquid crystal

In this article we disclose a method to fabricate a polarization independent dual frequency liquid crystal (DFLC) Fresnel lens (FL) with relatively high efficiency. The switchable FL is based on a patterned hybrid photo-aligned nematic (HPAN) DFLC cell assembled by one substrate providing the homeotropic anchoring and the other one providing the in-plane patterned alignment, with mutually orthogonal easy axis in the neighboring alignment domains, which has been prepared by means of two-step photo-alignment technique. Due to the electro-optical properties of dual frequency LC, the proposed HPAN DFLC FL, which manifests relatively high diffraction efficiency of 38.5% and low driving voltage, has the possibility of achieving a fast response by alternatively switching between high frequency and low frequency electric fields. Thus, with an excellent rewritable performance leading to the alternative focus/defocus switch, the HPAN DFLC FL would definitely be practical and promising in many modern devices.

Simulation of electrically controlled nematic liquid crystal Rochon prism

AbstractOperation of an electrically controlled beam steering device based on Rochon prism made by use of nematic liquid crystal is modelled numerically. Deflection angles and angular distribution of light intensity in the deflected beam are calculated. Dynamics of the device is studied. Advantage of application of dual frequency nematic liquid crystal is demonstrated. Role of flexoelectric properties of the nematic is analyzed.

Simulation of a symmetric optical response from a hybrid-aligned structure of a dual-frequency nematic liquid crystal

To create a hybrid-aligned structure of a dual-frequency nematic liquid crystal (LC), we have obtained an approximate analytical solution of the system of equations that describes the dynamics of reorientations of the director under the action of a control electric signal of an arbitrary shape. Formulas obtained have been used to simulate a symmetric optical response of the LC structure for a sinusoidal electric pulse. It has been shown that, in terms of the used approximations, the results of the analytical calculation agree well with results of computer simulation and with experiment in the case of small deformations of the LC layer.

Two-step switching in dual-frequency nematic liquid crystal mixtures

The so-called dual-frequency nematic mixtures are very promising components for applications in fast operating devices. Compared with classical nematics, they exhibit positive or negative anisotropy of the electric permittivity depending on the applied frequencies of an external electric field. Owing to this property, an overall switching process from planar to homeotropic orientations, and vice versa, can be shortened by using the electric field with two different frequencies. Electro-optical switching characteristics of transmission versus time as a function of applied voltage were obtained for two different dual-frequency mixtures in twisted nematic cells. For one of the investigated mixtures, unusual decrease in the light transmission at switching from the homeotropic to planar orientation at threshold voltage was observed. The switching process apparently occurs in two steps. The mechanism of the two-step switching at twisted dual-frequency nematic structures was discussed. The explanation of the switching mechanism takes into account the influence of the electric field with different frequencies on molecules with transverse and longitudinal dipole moments. Moreover, molecular structure of compounds constituting the mixtures was analyzed. Additionally, response times of the switching driven with low and high frequency pulses were shown. This work helps to understand the molecular interaction and electro-optical switching in the dual-frequency nematic liquid crystals.

Simulation of frequency dependence of dielectric susceptibility for description of director reorientation dynamics of nematic LC

Simulation of the liquid crystal behavior in an external electric field allows to determine the characteristics of the LC device for specific control signal. The reorientation of director tilt angles is described by the Frank-Oseen elasticity theory, by the Leslie-Ericksen hydrodynamics theory and also by Maxwell system of electromagnetic equations. Due to the fact that it is difficult to obtain analytical solution of dependencies polar and azimuthal angles on time it is necessary to use numerical methods. The numerical solution allows to determine the characteristics of the LC device for specific control signal, but it does not allow to solve the inverse problem. It is determining the necessary parameters of the control signal. To solve this problem, we obtained an approximate analytical solution of the dynamics equations of the director reorientation under the electric field for arbitrary values of the elastic constants and boundary conditions. The results of numerical simulation of the director reorientation under the electric field in hybrid-oriented structure of a dual-frequency nematic liquid crystal were obtained. It is shown that results of analytical solution are in good agreement with the results of computer simulation and experiment in case of small deformations of the LC layer and used approximations.