We have investigated the nonlinear optical interaction of uniform and kink states of a nematic and a ferrofluid-doped nematic (ferronematic) liquid crystal with an incident laser field. We find that the transition between the permitted... more
We have investigated the nonlinear optical interaction of uniform and kink states of a nematic and a ferrofluid-doped nematic (ferronematic) liquid crystal with an incident laser field. We find that the transition between the permitted uniform oreintational states of these systems is of first order in the case of nematics, and of second order in the case of ferronematics. In the latter case we also find the phenomenon of reentrance. We find new kink states in a magnetic field with topological winding different from π in the case of nematics, and 2π in the case of ferronematics. In ferronematics, due to grain segregation the phase diagrams for uniform and kink states are entirely different. In these systems we find a first or second order structural transformation from a single kink into a pair of kinks. Further, we obtain a rich variety of kink states as the intensity of the laser field is varied.
Discovery finds its roots in a desire to investigate the unknown. This notion of discovery defines the very interaction man has with nature; airplanes have sought to be the new birds, submarines have plunged deeper than any whale and... more
Discovery finds its roots in a desire to investigate the unknown. This notion of discovery defines the very interaction man has with nature; airplanes have sought to be the new birds, submarines have plunged deeper than any whale and solar cells have become the rightful heirs of plants. What happens though when discovery pertains to a system never before observed? When science is not content in merely imitating nature, it goes ‘beyond’ nature, inventing its own ‘meta’ realm. In this paper, a brief history of metamaterial research will act as a preface to a more detailed analysis of optical metamaterials and their shortcomings, highlighting the major issues when trying to find practical implementations when approaching visible wavelengths. Solutions to problems such as losses and instability are suggested via the introduction of gold nanoparticles coated with organic aliphatic chains in Liquid Crystal devices; these structures act as precursors to creating tailor-made metamaterial structures and show much promise in a diverse range of fields due to their versatility.
We demonstrate an electrically-controlled liquid crystal (LC) core waveguide, using 4-cyano-4 -pentylbiphenyl (5CB) nematic liquid crystal, fabricated on a glass substrate. A negative photoresist, AZ15nXT layer was coated on the glass... more
We demonstrate an electrically-controlled liquid crystal (LC) core waveguide, using 4-cyano-4 -pentylbiphenyl (5CB) nematic liquid crystal, fabricated on a glass substrate. A negative photoresist, AZ15nXT layer was coated on the glass substrate to realize a channel waveguide of core thickness 4.8 μm. The LC core waveguide exhibits strong differential attenuation for propagation of the TE- (horizontal) and TM-like (vertical) polarizations of light. The experimental results show that the output power suddenly drops down by several dB at an applied voltage of 1 V, for a waveguide of length 10 mm, due to the re-orientation of the LC molecules. The waveguide can be used as an optical switch, as well as an optical attenuator.
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... more
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.
Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic phase in order to... more
Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic phase in order to determine their refractive indices and order parameters using the Vuks method. These factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications and improvement. The temperature dependence of refractive indices, birefringence and order parameters in anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds have positive and large optical anisotropies that increase with decreasing temperature.
We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially... more
We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration 𝑝 below the percolation threshold. Simulations were carried for finite temperatures, where we varied 𝑝, interaction strength between LCmolecules, and impurities and external field𝐵. In the {𝐵, 𝑇} plane we determined lines separating short range— quasi long range and quasi long range—long range order. Furthermore, crossover regime separating external field and random field dominated regime was estimated. We calculated remanent nematic ordering in samples at 𝐵 = 0 as a function of the previously experienced external field strength 𝐵.
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... more
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 ...
Controlling the assembly of colloids in dispersion is a fundamental approach toward the production of functional materials. Nanocrystalline cellulose (NCC) is a charged nanoparticle whose colloidal interactions can be modulated from... more
Controlling the assembly of colloids in dispersion is a fundamental approach toward the production of functional materials. Nanocrystalline cellulose (NCC) is a charged nanoparticle whose colloidal interactions can be modulated from repulsive to attractive by increasing ionic strength. Here, we combine polarized optical microscopy, rheology, and small-angle scattering techniques to investigate (i) the concentration-driven transition from isotropic dispersion to cholesteric liquid crystals and (ii) salt-induced NCC phase transitions. In particular, we report on the formation of NCC attractive glasses containing nematic domains. At increasing NCC concentration, a structure peak was observed in small-angle X-ray scattering (SAXS) patterns. The evolution of the structure peak demonstrates the decrease in NCC interparticle distance, favoring orientational order during the isotropic− cholesteric phase transition. Small amounts of salt reduce the cholesteric volume fraction and pitch by a decrease in excluded volume. Beyond a critical salt concentration, NCC forms attractive glasses due to particle caging and reduced motility. This results in a sharp increase in viscosity and formation of viscoelastic glasses. The presence of nematic domains is suggested by the appearance of interference colors and the Cox−Merz rule failure and was confirmed by an anisotropic SAXS scattering pattern at q ranges associated with the presence of nematic domains. Thus, salt addition allows the formation of NCC attractive glasses with mechanical properties similar to those of gels while remaining optically active owed to entrapped nematic domains.
Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like objects interacting via the Lebwohl–Lasher interaction is... more
Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like objects interacting via the Lebwohl–Lasher interaction is considered. We test the applicability of predictions originally derived in cosmology and magnetism. In the first part we focus on coarsening dynamics following the temperature driven isotropic–nematic phase transition for different quench rates. The behavior in the early coarsening regime supports predictions made originally by Kibble in cosmology. For fast enough quenches, symmetry breaking and causality give rise to a dense tangle of defects. When the degree of orientational ordering is large enough, well defined protodomains characterized by a single average domain length are formed. With time subcritical domains gradually vanish and supercritical domains grow with time, exhibiting a universal scaling law. In the second part of the paper we study the impact of random-field-type disorder on a range of ordering in the (symmetry broken) nematic phase. We demonstrate that short-range order is observed even for a minute concentration of impurities, giving rise to disorder in line with the Imry–Ma theorem prediction only for the appropriate history of systems.
We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior. The obtained results are of interest for every randomly... more
We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior. The obtained results are of interest for every randomly perturbed system exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LC molecules are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a first order phase transition into the orientationally ordered nematic phase atT=Tcon lowering the temperatureT. The orientational ordering of LC molecules is perturbed by the quenched, randomly distributed rod-likeimpuritiesof concentrationp. Their orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC st...
We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. Theimpuritiesimposing orientational disorder were randomly spatially... more
We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. Theimpuritiesimposing orientational disorder were randomly spatially distributed with the concentrationpbelow the percolation threshold. Simulations were carried for finite temperatures, where we variedp, interaction strength between LC molecules, andimpuritiesand external fieldB. In the{B,T}plane we determined lines separating short range—quasi long range and quasi long range—long range order. Furthermore, crossover regime separating external field and random field dominated regime was estimated. We calculated remanent nematic ordering in samples atB=0as a function of the previously experienced external field strengthB.
Controlling the orientational ordering of liquid crystal molecules by confined surfaces is one of the main reasons for the widespread use of liquid crystals in the industry. The anchoring condition is an essential feature of the liquid... more
Controlling the orientational ordering of liquid crystal molecules by confined surfaces is one of the main reasons for the widespread use of liquid crystals in the industry. The anchoring condition is an essential feature of the liquid crystalline materials in vicinity of boundaries. As known, surface interaction energy in a confined liquid crystal can be studied in the framework of Rapini-Papoular model. However, for the uneven surfaces, other models, such as a model based on Fukuda theory should be used. The surface-like energy is not usually considered in energy calculations. In this work, we consider a nematic slab confined with two infinite surfaces and investigate the director field and anchoring effects in the total energy of the slab with a sinusoidal surface by taking into account this term of energy. We consider a surface with a grooved sinusoidal form and calculate the anchoring energy within the framework of Fukuda theory. We also obtain the dependence of energy on geometric parameters, such as grooves amplitude. By utilizing theoretical model proposed by Fukuda, we determine the director components and anchoring energy of the system under an external electric field. The variation of the director components and surface effects are discussed. It is shown that the anchoring energy has a maximum value in the absence of the field. Moreover, we find that the trend of anchoring energy variations is reduced due to the presence of an electric field.
We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior.The obtained results are of interest for every randomly... more
We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior.The obtained results are of interest for every randomly perturbed system exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LCmolecules are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a first order phase transition into the orientationally ordered nematic phase at𝑇 = 𝑇c on lowering the temperature𝑇.Theorientational ordering of LC molecules is perturbed by the quenched, randomly distributed rod-like impurities of concentration 𝑝. Their orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC structures in the symmetrybroken nematic phase are analyzed for two qualitatively different histories at 𝑇 ≪ 𝑇 𝑐. We demonstrate that, for a weak enough interaction among the LCmolecules and impurities, qualitatively different history-dependent states could be obtained. These states could exhibit either short-range, quasi-long-range, or even long-range order.
