Fast Electro-optic Response Research Articles

Orthoconic antiferroelectric liquid crystals

The ubiquitous liquid crystal display (LCD) is based on nematic liquid crystals (LCs) and has during 40 years developed from simple few digit displays into high-resolution flat-panel displays. A last and very important step towards the present TVs was the combination with thin film transistors that also led to the introduction of several new switching modes. Despite the enormous success of this technology, there is presently a renewed interest in LCs with faster electrooptic response, especially for future 3D vision display systems and possibly for field-sequential-colour (FSC) generation displays. Here, I focus on the so-called orthoconic antiferroelectric LCs (OAFLCs), which can provide the fast switching of conventional antiferroelectric LCs, but combined with a potentially much higher optical contrast. The reason is that the dark state of orthconic materials has a homeotropic optic axis, which makes the extinction ideally complete between crossed polarisers and independent of in-plane alignment fluctuations characteristic of all antiferroelectric LCs. The basic features and device physics issues of orthoconic LCs are reviewed and the most important remaining challenges to be met in terms of materials development are discussed in this article. Furthermore, a few examples of new applications made possible with the use of present and future orthoconic materials are given.

Porphyrins for Probing Electrical Potential Across Lipid Bilayer Membranes by Second Harmonic Generation

Neurons communicate by using electrical signals, mediated by transient changes in the voltage across the plasma membrane. Optical techniques for visualizing these transmembrane potentials could revolutionize the field of neurobiology by allowing the spatial profile of electrical activity to be imaged in real time with high resolution, along individual neurons or groups of neurons within their native networks.1, 2 Second harmonic generation (SHG) is one of the most promising methods for imaging membrane potential, although so far this technique has only been demonstrated with a narrow range of dyes.3 Here we show that SHG from a porphyrin-based membrane probe gives a fast electro-optic response to an electric field which is about 5–10 times greater than that of conventional styryl dyes. Our results indicate that porphyrin dyes are promising probes for imaging membrane potential.

Thin ferroelectric liquid crystal layers: mechanical stability and fast electro-optical response, connection between molecule design and surface properties

The storage of information after mechanical stimuli even without applied electrical field is based on the elastic properties of the ferroelectric liquid crystal (FLC) material and the anchoring energy. It is favourable for chiral molecules with extended length, such as 4,4′-di-substituted quaterphenyls and 4,4′-di-substituted quinquephenyls, which have in series-connected benzene rings, but not separated by bridging fragments. To obtain the required stability with a reasonable switching speed of about 50 μs 4,4′-di-substituted chiral quaterphenyls with two chiral centres on the same wing has been synthesised. Here, the switching is much faster compared to compounds with same quaterphenyl fragment, but only with one chiral centre.

Mesomorphic, optical, dielectric, elastic and viscous properties of multi-component isothiocyanato mixtures

Four nematic mixtures A, B, C and D, comprising alkylphenylethylphenyl isothiocyanates, alkylcyclohexylphenylisothiocyanates, biphenyl and terphenyl derivatives, respectively, were prepared and their physical properties investigated, including birefringence, static permittivity, dielectric anisotropy, splay elastic constant and rotational and bulk viscosity. All the mixtures exhibited a broad nematic range (∼100°C), moderate birefringence (Δn ∼0.23–0.26), positive dielectric anisotropy (Δε ∼ 8–10) and low rotational viscosity. The mixtures showed improved performance in birefringence and viscosity properties in comparison with the widely used commercial mixture E7. As far as display performance is concerned, mixture D was found to be the most valuable. A comprehensive study of the usefulness of the biphenyl and terphenyl derivatives as dopants in mixtures has also been conducted with a view to improving their physical parameters to give faster electro-optical response times in liquid crystal display devices. Application of these mixtures is envisaged in high-speed photonics and liquid crystal display devices.

Tunability of optical memory in ferroelectric liquid crystal containing polyvinylpyrrolidone capped Ni nanoparticles for low power and faster device operation

We demonstrate here tunability of optical memory and faster electro-optic response in ferroelectric liquid crystal dispersed with ferromagnetic nickel nanoparticles. Minute concentration of Ni induced an optical memory at a DC field as small as 2 V owing to the magnetoelectric effect of spin dependent screening. Memory retention period could be tuned by varying DC field and the concentration of nanoparticles as well. Furthermore, there is a remarkable improvement in response time (80%) and rotational viscosity of ferroelectric liquid crystal-Ni mixtures which is attributed to the coupling of strong intrinsic field of Ni nanoparticles with liquid crystal molecular director field.

Anomalously Large Bend Elastic Constant and Faster Electro-Optic Response in Soft Glassy Gels Formed by a Dipeptide

In this article, results of static/dynamic Freedericksz transformation and rheological studies on anisotropic thermoreversible gels formed by gelation of a nematic liquid crystal (NLC) with a monodisperse dipeptide are presented. Freedericksz transformation studies reveal a surprising result: the gel state possesses a very large Frank bend elastic constant value, order of magnitude higher than the sol state, whereas, the splay elastic constant shows relatively a small increase. The studies also demonstrate that the anisotropic gel to anisotropic sol transition can be tracked by simply monitoring the Freedericksz transformation. Other attractive features of the gel state include the vanishing of the undesirable backflow effect, and nearly an order of magnitude decrease of switching speed. Further, rheological studies bring out the soft glassy characteristics of these gels.

A Compact and Highly Efficient Silicon-Based Asymmetric Mach—Zehnder Modulator with Broadband Spectral Operation

An asymmetric Mach—Zehnder electro-optic modulator is demonstrated by using a silicon-based p-i-n diode embedded in compact 200 μm long phase shifters. The measured figure of merit VπL = 0.23 V ·mm shows highly efficient modulation by the device, and an open eye-diagram at 3.2 Gbit/s confirmed its fast electro-optic response. Integrated with the grating coupler, the device exhibits a broad operational wavelength range of 70 nm with a uniform 18 dB extinction ratio covering the C-band and part L-band of optical communication.

Anomalously large bend elastic constant and faster electro-optic response in anisotropic gels formed by a dipeptide

We report rheological, static, and dynamic Freedericksz transformation measurements on an anisotropic thermoreversible gel formed by gelation of a nematic liquid crystal (NLC) with a monodisperse dipeptide. The storage and loss modulii obtained from a low strain oscillatory shear experiment display that the material forms a weak anisotropic gel, and undergoes a sharp thermal transition to an anisotropic sol state. Freedericksz transformation studies employing an electric field for the reorientation of the molecules present a surprising result: the gel possesses a very large Frank bend elastic constant value, which is orders of magnitude higher than that for the high temperature sol state as well as that for the neat NLC used. On the other hand, the splay elastic constant shows relatively a small increase. Further, these elastic constants show systematic but nonlinear variation with the concentration of the gelator. Attractive features of the electro-optic switching when the sol transforms to the gel state are the vanishing of the undesirable backflow effect, and nearly an order of magnitude decrease in the switching speed. In both the gel and sol states the extracted rotational viscosities are comparable to the values of the neat NLC at corresponding temperatures. In contrast, the bulk dynamic viscosity is more than three orders of magnitude higher in the gel. The studies also demonstrate that the anisotropic gel to anisotropic sol transition seen in this weak gel can be tracked by simply monitoring the static or the dynamic Freedericksz transformation.

Mechanisms leading to fast relaxation of liquid crystal cells aligned with conductive polymers

Using nematic liquid crystal cells aligned by conductive polymers, like polyaniline or doped polypyrrole, a very fast electro-optic response is observed. We show that when a switch that interrupts the voltage across the cell is placed between the cell and the ground, the largest voltage drop is on the switch in its open position. The voltage distribution among the nematic cell and the switch is evaluated. The role played in this very fast electro-optic response of the ionic charges built on the interfaces together with the redox processes among the free charges in the polymer-liquid crystal interface is also described. For an asymmetric cell (only one side covered with a conductive polymer) a rectifying effect appears. In this case a circuital model is used to mimic the steplike behavior of the transmitted light during the relaxation of the system.

Effect of carbon nanotubes on response time of ferroelectric liquid crystals

We present the results of the fast electro-optic response of multiwalled carbon nanotubes (MWCNTs)-doped deformed helix ferroelectric liquid crystal (DHFLC). The fastening of the response in MWCNTs-doped DHFLC has been attributed to the decrease in rotational viscosity and increase in anchoring energy. The decrease in the former is due to the experience of the torque both by MWCNTs and DHFLC and perturbation of order parameter of the DHFLC while the increase in the latter is due to the pi-pi electrons stacking between the MWCNTs, DHFLC molecules, and alignment layers. The increase in conductance in doped cells has also been observed.

Wide Viewing Angle of Rubbing-Free Hybrid Twisted Nematic Liquid Crystal Displays

Low-voltage operation and fast response of liquid crystal displays (LCDs) can be achieved using a hybrid twisted nematic (HTN) cell, which is fabricated by applying the conventional twisted nematic (TN) cell. The devices unfortunately exhibit a narrow viewing angle in the same order as that of TN. It has been reported that an alignment layer made of hydroxypropyl cellulose (HPC) results in specific orientation; the director of the LC can be parallel to the substrate surface and random in the plane without any defects. The use of HPC as an alignment layer for an HTN cell results in a wide viewing angle as well as low-voltage operation and fast electrooptical response.

Electro-optical Response of Liquid Crystal Displays with Alignment Layers of a Cellulose Derivative

Low voltage operation and fast response of liquid crystal displays (LCDs) can be achieved by using a hybrid twisted nematic (HTN) cell, which is fabricated by applying the conventional twisted nematic (TN) cell. Unfortunately, the devices exhibit a narrow viewing-angle in the same order as that of TN. It has been reported that an alignment layer made of hydroxyl propyl cellulose (HPC) results in specific orientation; the director of the LC can be parallel to the substrate surface and random in the plane without any defects. The HTN cells with HPC as an alignment layer afford wide-viewing-angle as well as low voltage operation and fast electro-optical response.

34.2: Self‐Compensating, Quasi‐Homeotropic Liquid Crystal Device

AbstractWe present a quasi‐homeotropic liquid crystal device that is configured to suppress disclination line defects and to self compensate for in‐plane optical retardation and thereby exhibit an extremely high contrast ratio and fast electrooptic response.

Electro-optical response of the in-plane switching liquid crystal device fabricated using two-easy-axes substrate

We investigate the electro-optical response of an in-plane switching (IPS) liquid crystal (LC) device, which is fabricated using the two-easy-axes substrate. The two-easy-axes substrate is fabricated by slightly rubbing the substrate in two different directions. Experimental results indicate that the IPS LC device fabricated using the two-easy-axes substrate has a lower threshold voltage and a faster response time than the traditional IPS LC device, which is fabricated using the unidirectionally rubbed substrate. The weak anchoring condition and the anchoring strengths in two different rubbing directions on the substrate contribute to the fast electro-optical response of the IPS LC device.

Enhanced performance in traveling-wave electroabsorption Modulators based on undercut-etching the active-region

A novel traveling-wave electroabsorption modulator (TWEAM) based on undercut-etching-active-region waveguide (UEAW) is proposed and demonstrated. By selectively wet-etching the InGaAsP from InP to reduce InGaAsP active region, the waveguide can reduce parasitic capacitance with high conductivity in n-InP and p-InP cladding layers. In comparison with conventional ridge-waveguide (RW) TWEAM, 3 dB lower optical-insertion-loss, at least 6 dB higher in radio-frequency-link gain (dc to 40 GHz), and faster electrooptical response (3-dB bandwidth of 25 GHz at 50 /spl Omega/-termination for UEAW and 15 GHz for RW) are obtained in UEAW-TWEAM. A 10-Gbs/s operation with low swing voltage of 0.6 V has been achieved in UEAW-TWEAM, a 3.2-dB enhancement over RW-TWEAM. It indicates the tradeoff in designing electroabsorption modulators can be greatly released.

Dielectric Properties and Electro-optic Characteristics of TN-LCDs Doped with Metal Nanoparticles Exhibiting Frequency Modulation Response Accompanying Fast Response

ABSTRACT TN-LCDs doped with metal nanoparticles such as Pd, Ag, and Au are shown to exhibit a frequency modulation response together with the ordinary root-mean-square response to operating voltages. These devices are called FM-LCDs. The frequency range in this phenomenon spreads from several tens Hz to several thousands Hz depending on the materials of nanoparticles, their diameters, and their concentrations. The central frequency of each FM-LCD almost coincides with the dielectric relaxation frequency. FM-LCDs shows a sensitivity to the high frequency component of the driving voltage, this in turn gives rise to a fast electro-optic response in ms and sub-ms order. The EO effect of the FM-TN-LCD effect is explained based on the Maxwell and Wagner effect and it is shown that the metal nanopatricles have the effective electrical conductivity of σ2 = 6.0 × 105S/m that is about 100 times smaller that of metal Ag.

Morphological Studies of Holographically Formed Polymer Dispersed Ferroelectric Liquid Crystals Using Elevated Temperature Exposure

ABSTRACT In this contribution we describe a method of controlling the polarization dependence in holographically formed polymer dispersed liquid crystals using a ferroelectric liquid crystal mixture and a diacrylate monomer. By elevating the temperature of the prepolymer and liquid crystal mixture prior to the holographic exposure, we can control the polymer morphology and therefore the resulting alignment of the ferroelectric liquid crystal. The two distinct polymer morphologies that we manufacture and investigate are droplet and channel like. We investigate these grating structures using optical polarizing microscopy and scanning electron microscopy. We support our polarization dependence results with electro-optic characterization to see if ferroelectrics have added benefits of faster electro-optic response verses nematics in holographically formed polymer dispersed liquid crystal geometries.

金属ナノ粒子添加TN-LCDにおける高速度応答特性:オーバードライヴ効果とFETスウィッチング特性

金属ナノ粒子添加TN-LCDにおける高速度応答特性:オーバードライヴ効果とFETスウィッチング特性

Electro-Optical Bistability in Ferroelectric Liquid Crystal Switching Devices for Use in Displays and Real-Time Holography

Liquid crystalline (LC) materials which exhibit the ferroelectric Sc* phase showed early promise for device applications because of their fast electro-optic response and potential long-term electro-optical bistability. In combination these features should allow fast line at a time addressing and therefore potentially large passive matrix displays. Bistability should also facilitate DC balancing without the need for inverse frame or blanking procedures used in active matrix addressed liquid crystal over silicon devices. Such a feature is very valuable in the application of these materials to telecom devices where signal interruption for DC balancing purposes is very problematic. Long term bistability is also important in realising active tiling based displays. However the realisation of acceptable bistability has not been straightforward. Partly as a result, commercial passive matrix displays have been dominated by super twisted nematic (STN) devices, which have a limited voltage selection ratio that decreases with the number of pixel rows addressed. High quality flat panel LC displays for laptop computers, computer monitors and television has been delivered using active matrix thin film transistor twisted nematic (TFT TN) technology. The first generation of commercially important ferroelectric liquid crystal displays (for near-eye and projection applications) have in fact utilised active matrix liquid crystal over silicon (LCoS) device structures, where the speed of the FLC electro-optic response permits the use of frame sequential methods for generating colour and greyscales in images. Here we discuss the problems encountered in achieving useful bistability within FLC devices, the new applications that would greatly benefit from true FLC bistability and some interesting new results from some siloxane oligomers which suggest new possibilities in achieving true bistability. We hope to highlight the need for a deeper understanding of this problem.

Fast Electrooptic Response Based on Defect Mode Switching in One-Dimensional Photonic Crystal Containing Nematic Liquid Crystal and Ferroelectric Liquid Crystal as a Defect Layer

Electrical tuning of defect modes in a one-dimensional photonic crystal has been demonstrated using a ferroelectric liquid crystal and a nematic liquid crystal as a defect layer in the periodic structure. The theoretical calculation based on a simple transfer matrix has been carried out, and the result has been found to agree with the experimental result. A high-speed electrooptic switching based on the tunable defect mode has also been demonstrated. The response time of the order of several tens microseconds in this defect mode switching has been realized at low driving voltage.