2D tunable graded index prism beam steering device based on nematic liquid crystals

Liquid crystal devices are being used in many non-display applications in order to construct small devices controlled by low voltage electronics without mechanical components. In this work, we present a novel liquid crystal device for laser beam steering. In this device the orientation of the liquid crystal molecules can be controlled. A change in the liquid crystal orientation results in a change of the refractive index. When a laser beam passes through the device, the beam will be deviated (Fig.1) and the device works a prism. The main difference between this device and a prism is that in the device the orientation profile of the liquid crystal molecules can be modified so that the laser beam can be deviated a required angle: the device is tuneable.

Photonic Logic Based on Molecular Reorientation of Nematic Liquid Crystals

A new type of photonic logic, based on the use of nematic liquid crystals is proposed. The system takes advantage of the refractive-index changes induced by laser beams. Examples of AND, OR and NOR functions are presented.

Optically Induced Modulation of a Laser Beam in Nematic Liquid Crystals Structures

In this paper we report the experimental results obtained when an He-Ne laser beam crosses an MBBA homeotropic sandwich structure and is modulated by the influence of another laser beam, in our case an Ar+ laser, crossing through the same region. We extend some results previously reported by us1 2 concerning the influence of the ratio of the diameters of the laser beams on the modulation characteristics. A theoretical model, based on the one reported in Ref6 , shows good agreement with the experimental results. If the Ar+ laser is intensity chopped, the resulting He-Ne diffracted image is also intensity modulated. The highest frequency observed has been 500 p. p. s.

Magneto-Optical deflector with nematic liquid crystals

In this paper we report a new metod for optical switching based on the magneto-optical properties of liquid crystal materials. In order to improve previous response times, we used a wedge structure.

Strong flexoelectric behavior in bimesogenic liquid crystals

In this paper, we demonstrate strong flexoelectric coupling in bimesogenic liquid crystals. This strong coupling is determined via the flexoelectro-optic effect in chiral nematic liquid crystals based on bimesogenic mixtures that are doped with low concentrations of high twisting power chiral additive. Two mixtures were examined: one had a pitch length of p∼300nm, the other had a pitch length of p∼600nm. These mixtures exhibit enantiotropic chiral nematic phases close to room temperature. We found that full-intensity modulation, that is, a rotation of the optic axis of 45° between crossed polarizers, could be achieved at significantly lower applied electric fields (E<5Vμm -1) than previously reported. In fact, for the condition of full-intensity modulation, the lowest electric-field strength recorded was E=2Vμm-1. As a result of a combination of the strong flexoelectric coupling and a divergence in the pitch, tilt angles of the optic axis up to 87°, i.e., a rotation of the optic axis through 174°, were observed. Furthermore, the flexoelastic ratios, which may be considered as a figure-of-merit parameter, were calculated from the results and found to be large, ranging from 1.3to2C/Nm for a temperature range of up to 40°C. © 2006 American Institute of Physics.

Electro-optic characteristics of a transparent nanophotonic device based on carbon nanotubes and liquid crystals.

We present electro-optic characteristics of a transparent nanophotonic device fabricated on quartz substrate based on multiwall carbon nanotubes and nematic liquid crystals (LCs). The nanotube electrodes spawn a Gaussian electric field to three dimensionally address the LC molecules. The electro-optic characteristics of the device were investigated to optimize the device performance and it was found that lower driving voltages were suitable for microlens array and phase modulation applications, while higher driving voltages with a holding voltage can be used for display-related applications.

Bimesogenic liquid crystals: New materials for high performance flexoelectric and blue phase displays

In this paper we will describe new bimesogenic nematic liquid crystals that have high flexoelectro-optic coefficients (e/K),of the order of 1.5 CN 1 m-1, high switching angles, up to 100° and fast response times, of the order of 100μs or less. We will describe devices constructed, using the ULH texture that may be switched to the optimum angle of 45° for a birefringence based device with the fields of 4Vμm-1 over a wide temperature range. Such devices use an "in plane" optical switching mode, have gray scale capability and a wide viewing angle. We will describe devices using the USH or Grandjean texture that have an optically isotropic "field off" black state, uses "in plane" switching E fields, to give an induced birefringence phase device, with switching times of the order of 20μs. We will briefly describe new highly reflective Blue Phase devices stable over a 50V temperature range in which an electric field is used to switch the reflection from red to green, for example. Full RGB reflections may be obtained with switching times of a few milliseconds. Finally we will briefly mention potential applications including high efficiency RGB liquid crystal laser sources. © 2006 SID.

The limits of flexoelectricity in liquid crystals

The flexoelectric conversion of mechanical to electrical energy in nematic liquid crystals is investigated using continuum theory. Since the electrical energy produced cannot exceed the mechanical energy supplied, and vice-versa, upper bounds are imposed on the magnitudes of the flexoelectric coefficients in terms of the elastic and dielectric coefficients. For conventional values of the elastic and dielectric coefficients, it is shown that the flexoelectric coefficients may not be larger than a few tens of pC/m. This has important consequences for the future use of such flexoelectric materials in devices and the related energetics of distorted equilibrium structures. © 2011 Author(s).

Spontaneous induction of the uniform lying helix alignment in bimesogenic liquid crystals for the flexoelectro-optic effect

Using in-plane electric fields, the electrical induction of the uniform lying helix (ULH) alignment in chiral nematic liquid crystals is reported. This process permits spontaneous induction of the ULH alignment to give an in-plane optic axis, without the need for complex processing. Flexoelectro-optic switching is subsequently obtained by holding the in-plane electrodes at a common voltage and addressing via a third, plane-parallel electrode on a second, or upper, substrate to give a field across the device in the viewing direction. For this device, in optimized bimesogenic materials, we demonstrate full intensity modulation and sub-millisecond response times at typical device temperatures. © 2012 American Institute of Physics.

Increasing the flexoelastic ratio of liquid crystals using highly fluorinated ester-linked bimesogens.

We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.

Open-cell photoacoustic investigation of the thermal effusivity of liquid crystals

We report the use of an open photoacoustic cell configuration for the evaluation of thermal effusivity of liquid crystals. Initially, the method is calibrated using water and glycerol as transparent liquid samples, and the role of thermal conductivity of these liquids on the photoacoustic signal amplitude is discussed. To demonstrate the application of the present method for the evaluation of thermal effusivity of liquid crystals, we have used certain multicomponent nematic liquid crystal mixtures, namely BL001, BL002, BL032, and BL035. Each of these liquid crystal mixtures contains four to nine components and are primarily based on the cyanobiphenyl structure. The measured values of thermal effusivity of BL001 and BL002 were found to be almost the same, but differ from those of BL032 and BL035, which implies a difference in composition of the latter two from the former two mixtures.

Refractive indices and order parameters of two liquid crystals

Refractive indices have been measured throughout the nematic phase of 4-n-pentyl-4'cyanobiphenyl (5CB) and the smectic A and nematic phases of 4-n-octyl-4'-cyanobiphenyl (8CB). The Vuks and Neugebauer methods of calculating the order parameter are compared. Without knowledge of the molecular polarisabilities it is only possible to calculate a quantity proportional to the order parameter, and within this limitation it is found that the two methods give identical results. The order parameter is scaled using the extrapolation method suggested by Haller [14]. Using a suitable average of the refractive indices and the density data of Gannon and Faber [9], it is shown that the Lorentz-Lorenz relation is obeyed over a 2 % density range in 5CB.

Thermal tunability of photonic bandgaps in photonic crystal fibers selectively filled with nematic liquid crystal

We address the bandgap effect and the thermo-optical response of high-index liquid crystal (LC) infiltrated in photonic crystal fibers (PCF) and in hybrid photonic crystal fibers (HPCF). The PCF and HPCF consist of solid-core microstructured optical fibers with hexagonal lattice of air-holes or holes filled with LC. The HPCF is built from the PCF design by changing its cladding microstructure only in a horizontal central line by including large holes filled with high-index material. The HPCF supports propagating optical modes by two physical effects: the modified total internal reflection (mTIR) and the photonic bandgap (PBG). Nevertheless conventional PCF propagates light by the mTIR effect if holes are filled with low refractive index material or by the bandgap effect if the microstructure of holes is filled with high refractive-index material. The presence of a line of holes with high-index LC determines that low-loss optical propagation only occurs on the bandgap condition. The considered nematic liquid crystal E7 is an anisotropic uniaxial media with large thermo-optic coefficient; consequently temperature changes cause remarkable shifts in the transmission spectrums allowing thermal tunability of the bandgaps. Photonic bandgap guidance and thermally induced changes in the transmission spectrum were numerically investigated by using a computational program based on the beam propagation method. © 2010 SPIE.

Laser pulse shaping with liquid crystals

A method of unpolarized laser pulses shaping is reported. The basis of the method is the use of an hybrid optical bistable device with nematic liquid-crystals, similar to the one previously reported by us. A sample of the input light constrols, by an asymmetrical electronic comparator, a 1 x 2 electro-optical total switch. The output pulses are reshaped and maintain the same polarization properties as the input light. From triangular input light signals, symmetriacl and asymmetrical output pulses have been obtained. The minimum pulse width achieved was 0.1 msec. A representation of the output versus input light signals gives an hysteresys cycle in the asymmetrical case.

Dynamics Of Thermotropic Liquid Crystals Across The Isotropic-Nematic Transition And Their Similarity With Glassy Relaxation In Supercooled Liquids

No abstract.