Nematic Liquid Crystal Medium Research Articles

Modeling of laser generation in a Fabry–Pérot-Tamm structure with a nematic liquid crystal layer

In the presented work, the possibility of controlling laser generation using a nematic liquid crystal (NLC) in a hybrid layered structure consisting of a thin metal layer (Ag), a layer of NLC doped with a light-absorbing dye, and a distributed Bragg reflector (DBR) with a rectangular refractive index profile is theoretically studied. Spectral dependencies of the reflection, transmission, and absorption coefficients of light as well as the localization coefficient of the light field in NLC within the photonic bandgap of the DBR are obtained. Narrow dips in the reflection coefficient and peaks in the transmission coefficient are achieved due to the excitation of plasmons at the Ag-NLC interface. The dependence of the spectral position and magnitude of the plasmonic dips/peaks and the enhancement of the light field in the NLC medium on the thickness and orientation of the NLC layer as well as the impact of a light-absorbing dye doping are investigated. Theoretical calculations of the temporal dependencies of luminescence pulses for pumping pulses of different power settings (below, above, and at the threshold of laser generation) and different values of light absorption in the dye-doped NLC medium are performed, taking into account the peculiarities of the optical properties of the dye-doped NLC.

Nematic liquid crystal dispersed with two-dimensional functionalised graphene oxide (fGO): insights on improving the nematic ordering and reducing electro-optic response time

ABSTRACT Doping a nematic liquid crystal (NLC) with nanoparticles (NPs) of varied dimensions and properties to form LC-NP nanocomposites with improved electro-optic and dielectric properties has recently been a much-explored field of work. But still, NLC nanocomposites based on two-dimensional (2D) NPs have not been studied vividly because of the high tendency of 2D NPs to aggregate together. In the present work, we have studied the effect of incorporating hexadecanethiol functionalised graphene oxide (fGO), a 2D NP, in a conventional NLC medium under varying doping concentrations. We found that at a lower doping concentration (⁓10−4 wt.%), the NLC nanocomposite showed a substantial increase in dielectric anisotropy (Δε) and birefringence (Δµ) values as compared to pure NLC sample. A significant decrease (⁓50%) in ionic conductivity as well as reduced electro-optic response time (⁓75%) was also observed in this NLC nanocomposite. As the fGO doping concentration was increased (⁓10−2 wt.%), a reverse trend was observed for Δε, Δµ and response time values, which declined sharply with increasing doping concentration. Hence, the present work highlights two different working mechanisms at low- and high-doping concentrations of 2D NPs.

Deterministic topological defects and quantum dot assembly in a nematic liquid crystalline medium

Liquid crystal (LC) materials exhibit interesting electro-optic switching and molecular ordering properties. Furthermore, the addition of chemically compatible active emitters such as core-shell quantum dots (QD) in a LC medium allows optical as well as dielectric tuning in an anisotropic, reconfigurable ordered medium. Order in a nematic LC phase is characterized by an orientational order parameter. In this work, we demonstrate the use of patterned substrates to generate arrays of integer topological defects in a nematic LC medium doped with cadmium selenide (core) cadmium sulfide (shell) core–shell QDs. We demonstrate the formation of metastable air-pillar-induced integer topological defects (TDs) in relatively thinner 9 μm LC sandwich cells, and the formation of field-induced TDs in thicker (25 μm) cells. Simultaneously, the self-assembly of core-shell QDs into square arrays on the patterned substrates is discussed, highlighting potential electro-optic device applications. The surfactant hexadecyltrimethylammonium bromide (CTAB) is found to play a significant role in LC TD formation as well as QD spatial organization at the optimized concentration. Self-assembly and ordering of single- and multi-component LCs within structured devices is a highly relevant problem for modern optoelectronic devices. This work opens new possibilities for classical as well as quantum light sources which require spatially ordered optical emitters in a reconfigurable dielectric medium at a micron-scale.

Enhanced Mie resonance in a low refractive index colloidal metamaterial aided by nematic liquid crystal

We report studies on a colloidal metamaterial system in which sub-micron-sized (~530 nm) dielectric particles (SiO2) are dispersed in a nematic liquid crystal. Despite the low refractive index (~1.45) of the particles, Mie resonances are enhanced in the system due to the anisotropic nature of the host medium, as evident from the UV–Vis extinction spectra. The resultant improvement in light confinement and the interference between the dipolar Mie resonant modes give rise to forward directional scattering. Further, the Dark Field Hyperspectral Imaging and Heterodyne Near Field Scattering experiments demonstrate that the forward scattering intensity can be tuned by switching the refractive index of the liquid crystal medium between its anisotropic limits achieved by the application of an ac electric field. The extinction spectra with the deconvoluted resonance modes calculated using Mie theory and the Finite Elements Method (FEM) analysis support the experimental findings. The FEM calculations also reveal the formation of photonic nanojet from the particle. The enhanced Mie resonance and tunable forward scattering in the low index SiO2 particles induced by the nematic liquid crystal medium are highly promising for bio-compatible nano-photonic applications.

Effect of cobalt oxide nanoparticles on dielectric properties of a nematic liquid crystal material

Recently nanoparticles (NPs) are extensively explored as the dopant in liquid crystal (LC) materials because of their phenomenal properties such as good guest–host interaction and high surface to volume ratio. Keeping this motivation into mind, we have observed the influence of newly synthesized cobalt oxide (CoO) NPs on the dielectric and optical properties of nematic LC (NLC) namely 5 CB. For the observation of dielectric properties, we have performed dielectric spectroscopic measurements in the frequency range from 20 Hz to 1 MHz. The sample cell thickness-dependent studies have been performed to perceive the effect of applied field strength on dielectric parameters in presence of CoO NPs. Along with it, we have observed the alignment of the mesogens through the optical microscopy. So doping of such NPs serves various potential applications in the field of research as well as being proven to be cost-effective for industrial applications. In this article, we report the variation in dielectric properties of nematic liquid crystal (NLC) namely 5CB by the means of cobalt oxide nanoparticles (CoO NPs) doping. These CoO NPs may work as the ion absorbing agent in the NLC medium and can easily modify dielectric properties without influencing molecular alignment. Along with this, we have performed the LC sample cell thickness dependent studies of pure and CoO NPs doped 5 CB.

Evidence of Tunable Fano Resonance in a Liquid Crystal‐Based Colloidal Metamaterial

AbstractA colloidal metamaterial composite, realized by dispersing sub‐micrometer‐sized high refractive index dielectric resonators (selenium) in a nematic liquid crystal medium, exhibits electrically tunable Mie resonances in the optical regime. Darkfield hyperspectral imaging reveals that when the nematic liquid crystal (NLC) molecules reorient from the pristine planar state on application of an AC electric field, the scattered image from the particle splits into two, owing to the birefringence of the NLC medium. At higher voltages, a doughnut‐shaped scattering pattern is obtained, indicating the occurrence of Fano resonance. The analysis of the darkfield scattering spectra based on the “Multi‐pole Fano interference model” confirms the presence of Fano resonance arising due to the interference between the broad electric dipolar and narrow electric quadrupolar modes. With increasing voltage, the value of Fano parameter q decreases and approaches unity corresponding to an ideal Fano shape. Thus, the inherent optical anisotropy of the NLC medium and Fano coupling between the Mie resonant modes lead to highly reversible changes in the far‐field scattering pattern with potential applications in refractive index sensing, wave‐guiding, etc.

Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core-Shell Columnar Liquid Crystals.

Recently, we discovered a modular synthetic approach for constructing core-shell columnar liquid crystals (LCs) by supramolecular polymerization in LC media. In the present work, we successfully confirmed that our modular synthetic approach has the potential to be widely extended to the development of multifunctional columnar LCs. Herein, we constructed the first core-shell columnar LC that was proved to be orientable by both electric and magnetic fields by the supramolecular polymerization of NODiskNH* in a nematic LC medium of 4-cyano-4'-pentyloxybiphenyl (5OCB). NODiskNH* is a chiral benzenetricarboxamide derivative bearing 2,2,6,6-tetramethylpiperidine 1-oxyl termini, which is known to form a helical supramolecular polymer via a triple hydrogen-bonding array. NODiskNH* alone formed a hydrogen-bonded liquid phase without any long-range structural ordering. However, a nematic LC medium of 5OCB, when mixed with NODiskNH* at a molar ratio of 1:3, underwent a "structural order-increasing" mesophase transition, affording an optically active single LC phase with a hexagonally arranged core-shell columnar geometry in a temperature range from 113 to 51 °C. Unprecedentedly, this core-shell columnar LC can orient its columns both electrically and magnetically, resulting in unidirectional columnar ordering.

Giant light deflection via electro-mechanical modulation of liquid crystals

Liquid crystals (LCs) are matter with fluidity and anisotropy and have been used in various electro-optic devices for their capability to modulate the refractive index by voltage. Here, we show that LCs are capable of electro-mechanically modulating light to cause giant light deflection at low voltages (exceeding 64° at 1.0 V). We use a composite material where polymerized cholesteric LC particles that show optical Bragg reflection float in a nematic LC medium. The polymer-particles are elastically coupled with the host director through their surface molecular anchoring and rotate from a face-on to side-on configuration at the Frederik transition. Rigid-body rotation of the reflection plane causes light deflection, which is well reproducible and can be modelled theoretically. Our findings demonstrate the capability of LCs as a micro-electrical-mechanical system platform, which are potentially useful for large-area light-controlling applications.

Improvement of orientational order and display parameters of liquid crystalline material dispersed with single-wall carbon nanotubes

Composites were prepared by inserting Single-Walled Carbon Nanotubes (SWCNTs) in the Nematic Liquid Crystal (NLC) to fasten the reorientation dynamics of the system under the electric field. Dispersion of SWCNTs in NLC medium enhanced the orientational order and consequently the display parameters of composite systems improved. Various display parameters such as orientational order, dielectric anisotropy, threshold voltage and splay elastic constant were explored for pure and composite systems.

Suspensions of titania nanoparticle networks in nematic liquid crystals: rheology and microstructure

We study the influence of confinement on the rheology and structure of nematic liquid crystals (NLCs). NLCs get confined in networks of titania (TiO2, primary particle size = 21 nm) nanoparticles in suspensions of TiO2 and NLC, N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Suspensions with TiO2 nanoparticle volume fraction (ϕ) of 0.006–0.017, form viscoelastic solids with low elastic modulus (G′) of 101 Pa–102 Pa and short relaxation times. Increase in TiO2 nanoparticle ϕ leads to a rise in G′ with TiO2 nanoparticles forming a percolating network at a critical volume fraction (ϕ c) = 0.023, and G′ of ~103 Pa. TiO2/MBBA NLC suspensions at and above ϕ c = 0.023 show G′ ~ ω x−1 scaling, where ω is the angular frequency and the minimum in loss modulus (G′′) with ω. The effective noise temperature, x decreases and approaches 1 with the increase in the TiO2 nanoparticle ϕ from 0.023–0.035, is indicative of an increase in the glassy dynamics. Through the polarized light microscopy and differential scanning calorimetry experiments, we propose that the progressive addition of TiO2 nanoparticles introduces a quenched random disorder (QRD) in the NLC medium which disturbs the nematic order. This results in metastable TiO2/MBBA NLC suspensions in which NLC domains get confined in the network of flocs of TiO2 nanoparticles. We also show that the salient rheological signatures of soft glassy rheology develop only in the presence of NLC MBBA and are absent in the isotropic phase of MBBA.

Spontaneous electrorheological effect in nematic liquid crystals under Taylor-Couette flow configuration

Electrorheological (ER) characteristics of Nematic Liquid Crystals (NLCs) have been a topic of immense interest in the field of soft matter physics owing to its rheological modulation capabilities. Here we explore the augmentation in rheological characteristics of the nematic fluid confined within the annular region of the concentric cylindrical space with an Electrical Double Layer (EDL) induced at the fluid-substrate interface due to certain physico-chemical interactions. Using a Taylor-Couette flow configuration associated with an EDL induced at the inner cylinder wall, we show that a spontaneous electrorheological effect is generated owing to the intrinsic director anisotropy and structural order of complex nematic fluids. We seek to find the enhancement in torque transfer capability due to the inherent electrorheological nature of the nematic medium, apart from exploiting the innate nature of such homogeneous media to remain free of coagulation, a fact which makes it an excellent candidate for the applications in microfluidic environment. Our analysis reveals that with stronger induced charge density within the EDL, the apparent viscosity enhances, which, in turn, augments torque transfer across the concentric cylinder. The velocity profile tends to flatten in comparison to the classical circular Couette flow in annular geometry as one increases the surface charge density. We further observe a more pronounced ER effect for the nematic medium having larger electrical permittivity anisotropy. Besides the torque transfer qualifications, we also explore the distinct scenarios, wherein the same NLC medium exhibits shear thinning and shear thickening characteristics. The present configuration of the efficient torque transfer mechanism may be proficiently downscaled to micro-level and is relevant in the fabrication of micro-clutch and micro-dampers.

Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy

A photopolymerizable itaconic acid-based amphiphile (abbreviated as Ita3C12) consisting of a hydrophilic carboxylic acid, three alkyl tails, and a reactive vinyl function was newly designed and synthesized for the formation of automatic and robust vertical alignment (VA) layer of nematic liquid crystals (NLC). Since a hydrophilic carboxylic acid was chemically attached to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium were migrated toward the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer directly interacted with host NLC molecules and made them to automatically align vertically. Because of the reactive vinyl functions of Ita3C12 amphiphiles, it was possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers. The polymer-stabilized robust Ita3C12 VA layer exhibited a strong surface anchoring energy without generating any light scatte...

Optically isotropic liquid crystal media formulated by doping star-shaped cyclic oligosiloxane liquid crystal surfactants in twin nematic liquid crystals.

The formation of optically isotropic liquid crystal (LC) media has been investigated by doping the star-shaped LC molecular surfactants (SiLC) into the rod-shaped twin LC host molecules (DiLC). The experimental phase diagram was constructed on the basis of differential scanning calorimetry (DSC) and then a theoretical calculation was conducted through a combined Flory-Huggins (FH)/Maier-Saupe-McMillan (MSM)/phase field (PF) model to account for the experimental results. The phase diagram of the SiLC/DiLC mixtures revealed the broad coexistence regions such as smectic A + crystal (SmA1 + Cr2), liquid + crystal (L1 + Cr2), and liquid + nematic (L1 + N2) at the intermediate composition along with the narrow single phase crystal (Cr2), smectic (SmA1), and nematic (N2) regions. The morphologies and structures of these coexistence regions were further confirmed by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). At the 80/20 SiLC/DiLC composition, the optical anisotropy was induced under an alternating current (AC) electric field above its isotropization temperature. The formation of an optically isotropic LC medium in mixtures of the SiLC molecular surfactants and nematic LC host may allow us to develop new electro-optical devices.

Asymmetric Organic–Inorganic Hybrid Giant Molecule: Hierarchical Smectic Phase Induced from POSS Nanoparticles by Addition of Nematic Liquid Crystals

Spontaneous vertical alignment (VA) of a nematic liquid crystal medium, 4-cyano-4′-heptyloxybiphenyl (7OCB), was successfully achieved by directely adding a small amount of cyanobiphenyl monosubstituted polyhedral oligomeric silsesquioxane (POSS-CB) giant molecules [Kim, D.-Y.; Kim, S.; Lee, S.-A; Choi, Y.-E.; Yoon, W.-J.; Kuo, S.-W.; Hsu, C.-H.; Huang, M.; Lee, S. H.; Jeong, K.-U. J. Phys. Chem. C 2014, 118, 6300–6306]. The cyanobiphenyl moiety chemically attached to the pristine POSS improved the initial solubility with the 7OCB molecules and alleviated the macroscopic aggregates. However, the phase behavior and structural evolution of the finely tuned POSS-CB giant molecules with 7OCB are still open questions. Based on the thermal, microscopic, and scattering experiments, it was realized that the POSS-based giant molecules strongly interacted with host 7OCB at a molecular level and induced the layered structure. The POSS groups in the POSS-CB giant molecules were laterally close-packed to create the st...

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Modulational instability of optically induced nematicon propagation

We report the modulational instability (MI) analysis for the modulation equations governing the propagation of coherent polarized light through a nematic liquid crystal (NLC) slab, in the limit of low light intensity and local material response. The linear stability analysis of the nonlinear plane wave solutions is performed by considering both the wave vectors (k,l) of the basic states and wave vectors (K,L) of the perturbations as free parameters. We compute the MI gain, and the MI gain peak is reduced and the stable bandwidth is widened with the increase of the strength of the applied electric field. Further, we invoke the extended homogeneous balance method and Exp-function method aided with symbolic computation and obtain a series of periodic solitonic humps of nematicon profiles admitting the propagation of laser light in the NLC medium.

Time-Evolution Equations for Particle Dispersions in Nematic Liquid Crystal Media

A formal re-definition of the hydrodynamic equations for a compressible uniaxial nematic fluid in the presence of dispersed particles is presented, which is particularly amenable to computational treatment. Starting from the Parodi-Foster formulation of nematodynamics equations, it is shown that a functional formulation of the time evolution equations can be obtained which clearly separate conservative and dissipative terms. A simplified discretization scheme is also proposed, which leads to pseudo-particle representation of the fluid in combination with a standard description of the dispersed particles motion.

Steady-state and time-resolved spectroscopic investigations on intramolecular electron transfer processes within a synthesized methoxynaphthalene dyad by using a nematic liquid crystal medium

UV–vis, steady state and time-resolved spectroscopic investigations were made on photoinduced charge separation and thermal charge recombination processes involved within a novel synthesized dyad, 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA) where the donor 1-methoxynaphthalene (MNT) and the acceptor p-choloroacetophenone (PCA) moieties are connected by a short unsaturated olefinic bond. The measurements were made within the pseudo-ordered domain (just above nematic–isotropic (N–I) phase transition temperature, >308 K) of a nematic liquid crystal, 4-(n-pentyl)-4′-cyanobiphenyl (5CB). Results observed are compared with those obtained from the similar measurements in isotropic media. The charge separation and recombination rates remain more-or-less unchanged within the experimental error irrespective of the polarity of the environment, whether in pseudo-ordered domain ( ε S∼10.5) of a nematic liquid crystal 5CB or in highly polar isotropic medium ACN ( ε S∼37.5). The structural rigidity of the dyad MNCA having stable elongated form both in the ground as well as in the photoexcited states seems to be the reason for this unique behavior of solvent insensitivity. The theoretical predictions done by ab initio method density functional theory (DFT) with B3LYP/6-311 G (d, p) basis function correlate well with experimental observations of formations of only one stable elongated (E-type) conformer both in the ground and electronic excited state.

“Shear Acoustic Wind” Acting on Particles Embedded in a Liquid Crystal

We present here a theory of a specific force acting on the particles embedded in a liquid crystal (LC) if a “fast” (viscous) shear wave is excited in the LC layer. The origin of this force is very similar to the so-called “ponderomotive forces” acting on the electrons or bubbles in liquids in the presence of spatially non-homogeneous periodic fields (electric in the first case and acoustic in the second). We show that one can use this effect for manipulation, orientation through sedimentation, and separation of particles, including carbon nanotubes. This article is mostly focused on the case of elongated particles in a nematic liquid crystal medium. However, more general consideration is also presented.

Evidence for directed self-assembly of quantum dots in a nematic liquid crystal

Assembling quantum dots (QDs) into nanoscale configurations over macroscopic dimensions is an important goal to realizing their electro-optical potential. In this Rapid Communication, we present a detailed study of a pentylcyanobiphenyl liquid crystal (LC) and a CdS QD colloidal dispersion by probing the dielectric property epsilon and relaxation as a function of an applied ac electric field Eac. In principle, dispersing QDs in a nematic LC medium can direct the dots to align in nearly one-dimensional chainlike structures along the nematic director and these assemblies of QDs can be directed by external electric fields. In a uniform planar aligned cell, the Fréedericksz switching of the LC+QDs appears as a two-step process with the same initial switching field as the bulk but with the final epsilon value larger than that for an aligned bulk LC. The relaxation of epsilon immediately following the removal of Eac follows a single-exponential decay to its original value that is slower than the bulk but becomes progressively faster with increasing Eac, eventually saturating. These results suggest that the arrangement of the QDs is mediated by the LC.