Chiral Nematic Liquid Crystal Cells Research Articles

Emergence of disordered branching patterns in confined chiral nematic liquid crystals

Spatial branching processes are ubiquitous in nature, yet the mechanisms that drive their growth may vary significantly from one system to another. In soft matter physics, chiral nematic liquid crystals provide a controlled setting to study the emergence and growth dynamic of disordered branching patterns. Via an appropriate forcing, a cholesteric phase may nucleate in a chiral nematic liquid crystal, which self-organizes into an extended branching pattern. It is known that branching events take place when the rounded tips of cholesteric fingers swell, become unstable, and split into two new cholesteric tips. The origin of this interfacial instability and the mechanisms that drive the large-scale spatial organization of these cholesteric patterns remain unclear. In this work, we investigate experimentally the spatial and temporal organization of thermally driven branching patterns in chiral nematic liquid crystal cells. We describe the observations through a mean-field model and find that chirality is responsible for the creation of fingers, regulates their interactions, and controls the tip-splitting process. Furthermore, we show that the complex dynamics of the cholesteric pattern behaves as a probabilistic process of branching and inhibition of chiral tips that drives the large-scale topological organization. Our theoretical findings are in good agreement with the experimental observations.

Optical Imaging and Analytical Design of Localized Topological Structures in Chiral Liquid Crystals

We combine numerical modeling and analytical design techniques to study several of the most common localized topological structures in frustrated chiral nematic liquid crystal cells. An energy minimization procedure is applied to the lattice model to simulate the director field distributions. These distributions are also approximated using the suitably designed analytical ansatz. We present both simulated and approximated results for optical polarizing microscopy textures and different visualizations of director field structure such as distributions of the azimuthal director angle and isolines for the normal component of the director in coordinate planes. The ansatz correctly mimicked the geometry and optical properties of the solitonic structures under consideration.

Localized dissipative vortices in chiral nematic liquid crystal cells

Localized dissipative vortices in chiral nematic liquid crystal cells

Stable reflective state induced by a disturbed planar texture in surface-treatment-free chiral nematic liquid crystals.

Chiral nematic liquid crystals possess a one-dimensional periodic helical structure and are one of the oldest known materials with the ability of selective reflection of light. Their helix orientation, determining their optical properties, can be changed by a variety of stimuli, and it is also dominated by the surface treatment, ratio of the elastic constants and cell thickness. Here, we present a simple method to realize an angular independence reflective state, induced by a stable disturbed planar texture, in a surface-treatment-free chiral nematic liquid crystal cell. The scattering state caused by the defect-rich focal-conic texture can be electrically tuned to the reflective state from the disturbed planar texture in a very short time, and vice versa. These two optical conditions are both stable states in the null field until the next trigger. We find that the disturbed planar texture in the chiral nematic can provide a 100° viewing angle without reflected wavelength shift. The gray level of the reflected intensity can be tuned via application of the voltage pulses. Moreover, in this work, we discuss the effect of the chiral concentration on stabilizing the disturbed planar texture. When the chiral concentration is higher to induce the blue phases, the change in the texture of the ChNLCs after removing the strong electric field can stop at the disturbed planar texture with high reflectivity. In this work, the optical performance and the bistability based on the disturbed planar texture exhibits great potential for many applications, such as tunable filters, see-through/reflective displays and large-area smart windows.

Optimization of color and transmittance in a dye-doped chiral-nematic liquid crystal cell

Optimization of color and transmittance in a dye-doped chiral-nematic liquid crystal cell

Optimization of color and transmittance in a dye-doped chiral-nematic liquid crystal cell

Optimization of color and transmittance in a dye-doped chiral-nematic liquid crystal cell

Bistable switching between homeotropic and focal-conic states in an ion-doped chiral nematic liquid crystal cell

Operation of a light shutter usually requires that an electric addressing field is applied continuously to switch between a transparent and an extinct or opaque scattering state. To achieve low-power operation, a bistable light shutter, which consumes power only when switching between the two stable states, is preferred. Here, switching between the two stable states is induced with a short pulse rather than by applying a continuous field. In this work, we report bistable switching of an ion-doped chiral nematic liquid crystal between the transparent homeotropic and the scattering focal-conic states. Owing to ionic dopants, the use of complicated patterned electrodes or dual-frequency liquid crystals was not required for switching of the device. The light shutter exhibited an opaque state, which has higher scattering than previously reported bistable light shutters. Furthermore, doping of the reported device with a dichroic-dye enabled simultaneous control of light scattering and absorption. A new candidate for both smart window and see-through display applications is presented.

Alignment structures and diffraction properties of chiral nematic liquid crystal cells with periodically patterned photoalignment films

Liquid crystal (LC) cells with periodic alignment distributions were fabricated using chiral nematic LCs (N*LCs), which were prepared using mixtures of a nematic LC and a chiral dopant, along with photoreactive liquid crystalline polymer (PLCP) films. Periodic structures were formed by polarization holographic recording in the PLCP films. The director distribution in each cell depended on the ratio of chiral dopant present, i.e., the inherent helical pitch of the N*LCs. These periodic alignment structures with line defects in the LC grating cells were well explained on the basis of the elastic continuum theory of the N*LCs and the photoalignment effect of the PLCP films. The diffraction properties of the grating LC cells were also investigated using a polarized visible laser. The observed intensity and polarization states of the diffracted beams were consistent with theoretical ones calculated using the director distribution models. Our results clarify that the diffraction properties of the grating LC cells can be controlled by the helical pitch of the N*LCs.

Effects of chiral dopant on electro-optical properties of nematic liquid crystal cells under in-plane switching and non-uniform vertical electric fields

The electro-optical properties of the chiral nematic liquid crystal cell, driven by in-plane switching and non-uniform vertical electric fields, are investigated. The Bragg reflection, threshold voltage, and helical configuration are significantly related with the chiral dopant concentration. Through the driving-mode switching, two bistable helical structures without holding voltages are developed. The behavior of voltage- and temperature-dependent light reflection associated with the helical structure transition is also revealed. A fast bistable switching response (~5 ms) is successfully achieved by the three-terminal-electrode architecture. Further, the proposed cell shows a display time of over 6 hr in the unplugged power state.

Properties of nematicons in low-birefringent nematic liquid crystals

We investigate nonlinear self-focusing and nematicon generation in low birefringencenematic liquid crystals (NLCs), where n=0.04. are obtained for larger optical powers but they are more than in NLCs with standard birefringence, of the order of n=0.2 Moreover, in low-birefringent NLCs, polarization components exchange energy in a nonlinear regime. Such behaviour is not observed in typical NLCs, where ordinary and extraordinary waves tend to propagate independently due to large walk off.The results of our experimental rest are also compared with numerical simulations, using a fully vectorial BPM. Calculations and experimental results are in good qualitative and quantitative agreement. Full Text: PDF References G. Assanto and M. A. Karpierz, Nematicons: self-localised beams in nematic liquid crystals, Liq. Cryst. 36, 1161 (2009). CrossRef M. Peccianti and G. Assanto, Nematicons, Phys. Rep. 516, 147(2012). CrossRef M. Peccianti, G. Assanto, A. De Luca, C. Umeton and I. C. Khoo, Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells, Appl. Phys. Lett. 77, 7 (2000). CrossRef M. A. Karpierz, M. Sierakowski, M. Swillo and T. Wolinski, Self Focusing in Crystalline Waveguides, Mol. Cryst. Liq. Cryst. 320, 157 (1998). CrossRef U. A. Laudyn, M. Kwasny, M. A. Karpierz, Nematicons in chiral nematic liquid crystals, Appl. Phys. Lett. 94, 091110 (2009). CrossRef U. A. Laudyn, M. Kwasny, K. Jaworowicz, K. A. Rutkowska, M. A. Karpierz and G. Assanto, Properties of spatial solitons in chiral nematic liquid crystal cells, Photon. Lett. Poland 1, 7 (2009). CrossRef I. C. Khoo, Liquid Crystals, Second Edition, Wiley & Sons, New Jersey (2007). CrossRef A. Piccardi, A. Alberucci, G. Assanto, Soliton self-deflection via power-dependent walk-off, Appl. Phys. Lett.96, 061105 (2010). CrossRef F. A. Sala and M. A. Karpierz, Modeling of molecular reorientation and beam propagation in chiral and non-chiral nematic liquid crystals, Opt. Express 20, 13923 (2012). CrossRef A. Piccardi, M. Trotta, M. Kwasny, A. Alberucci, R. Asquini, M. Karpierz, A. D'Alessandro and G. Assanto, Trends and trade-offs in nematicon propagation, Appl. Phys. B 104, 805 (2011). CrossRef R. Dąbrowski, J. Dziaduszek and T. Szczucinski, Mesomorphic Characteristics of Some New Homologous Series with the Isothiocyanato Terminal Group, Mol. Cyst. Liq. Cryst. 124, 241 (1985). CrossRef

Density of photon states in dye-doped chiral nematic liquid crystal cells in the presence of losses and gain

We calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behavior of the DOS is also discussed for amplifying LC cells providing additional insight to the lasing mechanism of these structures.

Rotational Behavior of Stripe Domains Appearing in Hybrid Aligned Chiral Nematic Liquid Crystal Cells

The appearance of stripe domains is strongly affected by various cell conditions such as thickness, applied voltage, and surface alignment. In this work, we adopt hybrid surface alignment to investigate the appearance of stripe domains under asymmetric surface anchoring. It is observed that the stripe direction rotates continuously according to the cell thickness and applied voltage, which has potential as a unique diffraction grating. It is also found that the rotational response time is always several seconds for any size of cell, because a large area rotation is achieved by the sum of small poly domain rotations related to defect creation–extinction processes on their boundaries.

Nematicons in chiral nematic liquid crystals

We report on the experimental studies of the existence of spatial solitons called nematicons in chiral nematic liquid crystal cells. The low absorption allows us to observe soliton propagation at a distance of over a few millimeters in range. The results of our experiment also show that it is possible to create independent nematicons in different layers formed by a helical structure of the liquid crystals.

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.

Simple method for the calculation of the deformation profiles in chiral nematic liquid crystal cells with asymmetric pretilt

We present a one-dimensional simulation methodology for determining the liquid crystal director distribution in a cell for arbitrary pretilt angles on two glass substrate surfaces. The method is illustrated for cases of an asymmetric twisted nematic cell, a hybrid alignment nematic cell, and a bend alignment nematic cell.

Director deformation of a twisted chiral nematic liquid crystal cell with weak anchoring boundaries.

On the basis of a generalization of the Rapini-Papoular expression [J. Phys. (Paris) Colloq. 30, C4-54 (1969)] for the anchoring energy, the rigorous expressions for the threshold and saturation fields are derived analytically, in detail, for a field-controlled twisted chiral nematic slab with weak boundary coupling. The surface anchoring energy of the Rapini-Papoular type for a twisted nematic slab should be generally expressed as the interfacial energy per unit area for a two-dimensional deformation which is a nonlinear combination of the azimuthal and polar angles. Applying a variational calculation method for the two-dimensional problem to the total free energy, that is, the sum of the bulk energy and the surface energy, we derive general torque balance equations which describe the equilibrium deformation of the director.

General Threshold Properties of Liquid Crystal Slab With Weak Anchoring Boundaries

Abstract On the basis of the Rapini-Papoular anchoring energy unifying the azimuthal and polar angle anchorings, the general surface torque balance equations for field-controlled twisted chiral nematic liquid crystal cells with weak anchoring boundary coupling have been derived analytically, The relationship between the anchoring energy and the threshold field of Freedericksz transition has been discussed rigorously.

A 2*2 fiber optic switch using chiral liquid crystals

A nonchemical 2*2 optical switch for fiber optic applications is described. Using a chiral-nematic liquid-crystal cell and switchable nematic liquid-crystal waveplates, the authors measured -26-dB crosstalk between unselected fibers, a fiber-to-fiber insertion losses of 1.4 dB, and switching times under 40 ms with operation at 1318 nm using 62.5/125- mu m multimode fiber. The device features simple construction, low power requirements, and an optical bandwidth suitable for LED or laser sources, and it works equally well with multimode or single-mode fiber.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>