Negative Dielectric Anisotropy Research Articles

Transition between self-focusing and self-defocusing in a nonlocally nonlinear system

We reveal the relevance between the nonlocality and the focusing/defocusing states in nonlocally nonlinear media, and predict a novel phenomenon that the self-focusing/self-defocusing property of the optical beam in the nonlocally nonlinear medium with a sine-oscillation response function depends on its degree of nonlocality. The transition from the focusing nonlinearity to the defocusing nonlinearity of the nonlinear refractive index will happen when the degree of nonlocality of the system goes cross a critical value, and vise verse. Bright and dark soliton solutions are obtained, respectively, in the focusing state and in the defocusing state, and their stabilities are also discussed. It is mentioned that such a phenomenon might be experimentally realized in the nematic liquid crystal with negative dielectric anisotropy or in the quadratic nonlinear medium.

6‐1: Invited Paper: UBplus: New Solution for High Resolution TV

Higher resolution is the key trend for next generation TV applications. One challenge is keeping the transmittance while pixel density increases. UBplus technology using LC mixtures with negative dielectric anisotropy offers a unique solution to this challenge in a classic FFS cell layout. This paper summarizes the recent developments of liquid crystal mixtures for UBplus in terms of performance and reliability.

Effect of graphene oxide dispersion in nematic mesogen and their characterization results

Graphene oxide is dispersed in negative dielectric anisotropy nematic liquid crystal p-methoxybenzylidene p-decylaniline (MBDA) in 0.2 wt/wt%, 0.4 wt/wt%, 0.6 wt/wt%, and 0.8 wt/wt% concentrations (abbreviated as MIX 1, MIX 2, MIX 3 and MIX 4). The paper proposes techniques to characterize and fill in the sample holder nematic–graphene oxide composites. The textures, electro-optical, dielectric as well as optical properties at 54 °C in nematic phase are investigated. X-ray diffraction result shows the particle size of graphene oxide to be 10–60 nm. The zeta potential of graphene oxide is around − 35 mV and its absolute value decreases from 31.5 mV in pure MBDA to 0.56 mV in MIX 4. Polarizing microscopy results display that MBDA and its composites in unaligned cell exhibit poor contrast with non-uniformity in bright state whereas aligned ones exhibit good contrast with uniformity. The observation reveals that switch-on time practically remains constant, while switch-off time and overall response time reduces from 25.8% (MIX 1) to 83.67% (MIX 4) and 25.69–83.074%, compared to pure MBDA. Rotational viscosity increases from 2.59 to 18.18%. The threshold voltage and splay elastic constant in pure MBDA and its composites increase from 2 to 3 V and 7.25 to 8.12 pN. Reduction in dielectric parameters has been observed. The magnitude of $$~\varepsilon {'_\parallel }$$ and $$\varepsilon {'_ \bot }$$ reduces from 44.2 to 76.1% and 24.6 to 72.8%, which shows $$\varepsilon {'_ \bot }$$ is always greater than $$\varepsilon {'_\parallel }$$. Therefore, absolute dielectric anisotropy reduces from 1.49 to 79.10%. The dielectric relaxation frequency reduces from 4.3 × 105 Hz to 2.81 × 105 Hz without bias voltage and 6.13 × 105 Hz to 2.2 × 105 Hz with bias voltage. UV–Vis absorbance study discloses π–π* transition with absorbance increase from 4.44 to 57.18% and corresponding blue shift in wavelength (249–194 nm). 7.23–65.90% quenching in photoluminescence study has been revealed. FTIR spectra are recorded on wavenumber range from 655 to 3500 cm−1. This work has application in sensors, imaging functions, improved electro-optic display devices and colorful displays devoid of color filters.

Wide tunable laser based on electrically regulated bandwidth broadening in polymer-stabilized cholesteric liquid crystal

Electrically responsive photonic crystals represent one of the most promising intelligent material candidates for technological applications in optoelectronics. In this research, dye-doped polymer-stabilized cholesteric liquid crystals (PSCLCs) with negative dielectric anisotropy were fabricated, and mirrorless lasing with an electrically tunable wavelength was successfully achieved. Unlike conventional liquid-crystal lasers, the proposed laser aided in tuning the emission wavelength through controlling the reflection bandwidth based on gradient pitch distribution. The principal advantage of the electrically controlled dye-doped PSCLC laser is that the electric field is applied parallel to the helical axis, which changes the pitch gradient instead of rotating the helix axis, thus keeping the heliconical structure intact during lasing. The broad tuning range (∼110 nm) of PSCLC lasers, coupled with their stable emission performance, continuous tunability, and easy fabrication, leads to its numerous potential applications in intelligent optoelectronic devices, such as sensing, medicine, and display.

Optical and dielectric parameters of a laterally difluorinated phenylbicyclohexane liquid crystalline compound

The temperature variation of the refractive indices in the nematic range of a laterally difluorinated phenylbicyclohexane liquid crystalline compound was determined using the thin prism technique. Birefringence and effective geometry parameter of this LC compound was calculated from the measured refractive indices. Using a four-parameter model, the temperature variation of the refractive indices and birefringence was theoretically fitted. The compound is found to have low value of birefringence and high value of effective geometry parameter. Using refractive index data, three different methods were used in this work to obtain temperature dependence of order parameter, namely Vuks’ method, Haller approximation method, and method based on effective geometry parameter. All of them show the same trend. The temperature variation of dielectric permittivities of the compound was done at frequency 1 kHz by an LCR meter. The temperature variation of dielectric anisotropy of the compound was calculated from the experimentally measured values of dielectric permittivities. The compound shows moderate negative dielectric anisotropy.

Electro-optic quenching of nematicon fluctuations.

Employing a low-frequency external electric field bias in nematic liquid crystals with negative dielectric anisotropy, we demonstrate that the trajectory fluctuations of reorientational spatial solitons can be substantially reduced, improving confinement and polarization purity of the output beams.

Effect of flexoelectricity on the Frèedericksz transition in chiral nematics with negative dielectric anisotropy

It is shown theoretically that due to the flexoelectric effect the electric field driven Frèedericksz transition can take place in chiral nematic (cholesteric) liquid crystal with negative dielectric anisotropy. It is especially noteworthy, since such a transition in a plane parallel cell is impossible in the absence of flexoelectricity. The spatial inhomogeneity of the electric field is is taken into account in the free energy alongside with the traditionally considered terms, describing the orientation elasticity and the anchoring at the boundaries. The phase diagrams are plotted and the orientation structures above the threshold are analyzed basing on the free energy minimization. A comparison between the phase diagrams for liquid crystals with positive and negative anisotropy of dielectric permittivity is made. It was found, that for sufficiently high flexoelectric coefficients the orientational structure can be described by simple functions. In this case, as a result of the Frèedericksz transition, the orientation structure of the liquid crystal changes from planar helicoidal to a hybrid one. This gives rise to new opportunities for the design of switching devices.

Electro-thermally tunable reflective colors in a self-organized cholesteric helical superstructure

We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (λc) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium–tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state, which is subject to vertically applied voltage, should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, λc of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance–capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of λc in the entire visible spectrum is as low as 12 Vrms in a 5 μm thick cell at a frequency of 2 MHz. The proposed CLC cell, exhibiting a broad electrically tunable spectral range from the near-infrared to ultraviolet, holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors.

Stability of liquid crystal micro-droplets based optical microresonators

ABSTRACTLiquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.

Control of haze value by dynamic scattering in a liquid crystal mixture without ion dopants

We report a method to control the haze value by dynamic scattering in a liquid crystal mixture without ion dopants. The liquid crystal (LC) mixture consists of chiral smectic C (SmC*) and nematic (N) LC molecules. In the absence of an electric field, LC molecules in the SmC* phase are oriented by the neighboring vertically-aligned N-LC molecules so that the LC cell is transparent. When a vertical electric field is applied to the LC cell, N-LC molecules with negative dielectric anisotropy tilt down in a random direction parallel to the two substrates, whereas SmC*-LC molecules are rotated with the polarity reversal of the applied field. During the switching process, we observed dynamic scattering because the random orientation of N-LCs is maintained via the rotation of SmC*-LCs, resulting in a high-haze translucent state without the use of ionic dopants. The LC mixture exhibited a very high haze of 96.1% in the translucent state while maintaining the haze-free (<1%) transparent state owing to the use of an LC layer without a polymer structure.

Visualising the crossover between 3D and 2D topological defects in nematic liquid crystals

ABSTRACTThe topological properties of disclinations are quite different for liquid crystals in two dimensions (2D) or three dimensions (3D). In 2D, there are distinct types of disclinations with topological charges or winding numbers of any half-integer or integer. By contrast, in 3D, all half-integer disclinations are topologically equivalent to each other, and integer disclinations are not defects at all. In this study, we use numerical simulations to explore the crossover between 3D and 2D. We show that certain disclination lines between patterned surfaces can exist when the director field is free to rotate in 3D, but not when the director field is forced into the 2D plane (by an electric field applied to a liquid crystal with negative dielectric anisotropy). As a result, these disclinations are expelled from the liquid crystal.

Selective variable optical attenuator for visible and mid-Infrared wavelengths.

This work demonstrates a variable optical attenuator (VOA) using dynamic scattering mode (DSM) in ion-doped liquid crystals with negative dielectric anisotropy. The mechanism of attenuation comes from optical scattering, which is generated by the electrically induced instability of undulation of LC textures. Electric fields are applied to switch the initial transparent state of the designed VOA to scattering states, varying the transmittance. The electric field also changes the size of the scattering domain from the LC texture and causes the designed device to exhibit an ultra-broadband selective operation in a visible to mid-IR spectral range. Furthermore, the VOA can selectively block one visible or mid-IR wavelength of light while letting other light pass. Such a VOA has many superior optical switching properties, such as high on/off contrast, insensitivity to polarization, and spectral selectivity; therefore, it has the potential to be used in practical optical systems.

Nonelectric Sustaining Bistable Polymer Framework Liquid Crystal Films with a Novel Semirigid Polymer Matrix.

In this work, a bistable polymer framework liquid crystal (PFLC) thin film by thermal curing of epoxy monomers with two different thiols, a traditional flexible-structure thiol and a novel original rigid-structure thiol, has been successfully fabricated, combining a novel mixed morphology of polymer matrix and cholesteric liquid crystals with negative dielectric anisotropy. The polymer framework morphology has been formed by curing two types of epoxy monomers with two types of thiols, and the liquid crystals tend to be focal conic textures with large size domains at the initial state in the PFLC film so that it has a moderate light transmissivity at this state between the transparent state and the opaque state. Thus, the devices based on PFLC films can be switched reversibly between the transparent state and the opaque state by alternative electric field. In addition, the states can be sustained after the electric field is removed. The bistable memory effect comes from the anchoring effects of the polymer frameworks with a novel morphology in the microdomains of the PFLCs. Therefore, the optimized bistable PFLC film keeps its initial state without external electric field and any other energy consumption for a long time after altering the state by applying an instant electric field. The special polymer frameworks in the bistable PFLC films endow the films with excellent electro-optical properties and mechanical properties. The devices are energy-efficient and cost-saving and have great potential applications in energy-efficient reflective displays, electronic papers, writing tablets, and smart windows.

Tuning of Polarization-Dependent Whispering Gallery Modes in Grapefruit Microstructured Optical Fibers Infiltrated With Negative Dielectric Anisotropy Liquid Crystals

An electrically tunable whispering gallery mode microresonator based on microstructured optical fibers (MOFs) infiltrated with negative dielectric anisotropy liquid crystals (LCs) is proposed and experimentally demonstrated. Experimental results indicate that the second radial order mode of the MOF microresonator has stronger electric field response than the first radial order mode and the resonance dip for TE polarization mode is more sensitive to the applied electric field intensity in comparison with the TM polarization mode resonance dip. The Freedericksz transition threshold of the proposed MOF microresonator is experimentally found to be about $\text{2.0}\;{\text{V}}\mu {{\text{m}}^{ - 1}}$ . The electrically tunable microresonator integrated with negative dielectric anisotropy LCs is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.

Large-scale periodic pattern in homeotropic liquid crystals induced by electric field

ABSTRACTIn-plane periodic modulation of the director field of an initially homeotropic liquid crystal layer with negative dielectric anisotropy is studied in external electric field with time varying amplitude envelope. Periodically turning a low frequency electric field on and off the cell, an in-plane periodic modulation of the director develops with a length scale that is orders of magnitude larger than the cell gap and can be as large as couple of centimetres. Doping the liquid crystal layer with low concentration of dichroic dye allows easy mapping of the director. We show that the periodic pattern is a periodic arrangement of +1 and −1 point defects, so called ‘umbilics’. We argue that the in-plane direction of the director is governed by the local flow that accompanies the turn on and off of the electric field. The finite size and the shape of the cell, incompressibility constraint and the nature of the surface alignment material, all together with the flow, are ultimately responsible for the development of the observed periodic director modulation.

Light‐Driven Electrohydrodynamic Instabilities in Liquid Crystals

AbstractThe induction of electrohydrodynamic instabilities in nematic liquid crystals through light illumination are reported. For this purpose, a photochromic spiropyran is added to the liquid crystal mixture. When an electrical field is applied in the absence of UV light, the homeotropic liquid crystal reorients perpendicular to the electrical field driven by its negative dielectric anisotropy. Upon exposure to UV light, the nonionic spiropyran isomerizes to the zwitterionic merocyanine form inducing electrohydrodynamic instabilities which turns the cell from transparent into highly scattering. The reverse isomerization to closed‐ring spiropyran form occurs thermally or under visible light, which stops the electrohydrodynamic instabilities and the cell becomes transparent again. It is demonstrated that the photoionic electrohydrodynamic instabilities can be used for light regulation. Local exposure, either to drive the electrohydrodynamics or to remove them enables the formation of colored images.

Performance analysis and negative dielectric anisotropy behavior of nematic liquid crystal doped with newly synthesized 1-(p-chlorobenzyl)-5-nitrobenzimidazole cobalt complex

The study investigates electrical current and dielectric properties of liquid crystal (LC) mediums. The studied samples are pure E63 and di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride doped E63.The combination of 1-(p-chlorobenzyl)-5-nitrobenzimidazole (1) with metal salt (CoCl2·6 H2O) was prepared stable solid new di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride (2). Synthesized di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride was identified by 1H NMR spectroscopic technique and elemental analysis. Frequency dependence of the dielectric properties and dielectric anisotropy of LC sampleswas investigated using the admittance spectroscopy method (C-V and G/ω-V) in the frequency range of 100 Hz-10 MHz at room temperature. E63 coded nematic crystal was used as pure liquid crystal. The doping material concentration is ensured 1 wt % in E63. Dielectric constant (ε'), dielectric loss (ε"), dielectric loss tangent (tanδ) and real and imaginary parts of electrical modulus (M'and M") of the LC samples calculated. Also, dielectric anisotropy (Δε) versus frequency plots were obtained for both samples.

Normally transparent smart window based on electrically induced instability in dielectrically negative cholesteric liquid crystal

We demonstrate a normally transparent smart window based on a cholesteric liquid crystal with negative dielectric anisotropy. The window is electrically switchable between clear and diffuse states. The clear state exhibits a well-aligned planar cholesteric texture and is stable in the absence of an electric field, while the diffuse state is switched on when applying a field higher than the undulation instability threshold. The degree of translucency can be controlled by varying the field strength. When the applied field is removed, the smart window relaxes back to the clear state spontaneously. It is also found that a much faster diffuse–clear process can be stimulated by reducing the field below the instability threshold. The smart window presents itself a promising projector screen for augmented reality applications as it features single stable state, sub-second switching speed, and polymerization-free fabrication.

Enhanced reflection band broadening in polymer stabilized cholesteric liquid crystals with negative dielectric anisotropy

The photonic band gap of cholesteric liquid crystal with negative dielectric anisotropies can be broadened under DC electric voltages by polymer stabilization. The broadening is produced by the movement and aligning effect of the dispersed “structural” chiral polymer network. We carried out a systematic study of factors, such as the alignment layer, moisture and ionic additives, which affect the broadening. Our experimental results show that the degree of broadening is strongly dependent on the ion concentration in the polymer stabilized cholesteric liquid crystal. We observed a linear relation between the broadening and the ion density. By using the optimal material formulation, we achieve 3 folds broadening to cover the entire visible light region with low voltages.

Effect of Barium Titanate Particles on Dielectric and Electro-Optical Properties of a Smectic-a Liquid Crystal

The effect of submicron ferroelectric BaTiO3 particles on the dielectric and electro-optical properties of the smectic-A liquid crystal (LC) with a high negative dielectric anisotropy is investigated. It is shown that the addition of BaTiO3 particles with a weight amount of 1% reduces insignificantly the transverse dielectric permittivity component e⊥ of, but significantly increases the longitudinal dielectric permittivity component e// of the smectic-A LC. As a result, the anisotropy of the dielectric permittivity Δe = e// − e⊥ of the smectic-A LC decreases. The addition of BaTiO3 particles shifts the dispersion e⊥ toward lower frequencies. Both components of the electrical conductivity of LC colloid + BaTiO3 are an order of magnitude higher than of the pure LC. The threshold voltage of the homeotropic-planar transition of the colloid is twice smaller, and its velocity is 6 times higher in comparison with the pure LC. A simple model explaining qualitatively all results obtained is presented.