Switching Of Nematic Liquid Crystals Research Articles

All-optical switching of liquid crystals at terahertz frequencies enabled by metamaterials.

Nematic liquid crystals integrated with metallic resonators (metamaterials) are intriguing hybrid systems, which not only offer added optical functionalities, but also promote strong light-matter interactions. In this report, we show with an analytical model that the electric field generated by a conventional oscillator-based terahertz time domain spectrometer is strong enough to induce partial, all-optical switching of nematic liquid crystals in such hybrid systems. Our analysis provides a robust theoretical footing for the mechanism of all-optical nonlinearity of liquid crystals, which was recently hypothesised to explain an anomalous resonance frequency shift in liquid crystal-loaded terahertz metamaterials. The integration of metallic resonators with nematic liquid crystals offers a robust approach to explore optical nonlinearity within such hybrid material systems in the terahertz range; paves the way towards increased efficiency of existing devices; and broadens the range of applications of liquid crystals in the terahertz frequency range.

Inverse "guest-host" effect: ferroelectric nanoparticles mediated switching of nematic liquid crystals.

Liquid crystals are widely used as a host matrix to embed different materials: dyes, fullerenes, carbon nanotubes, various nanoparticles (metallic, semiconductor, ferromagnetic, ferroelectric). The usual approach is related to the so called "guest-host" effect: external electric (or magnetic) fields drive liquid crystals (host), and liquid crystals reorient embedded particles (guest). In this paper we report an experimental observation of the effect that is completely opposite to the classical "guest-host" phenomenon: ferroelectric nanoparticles being switched by an external field mediate the switching of liquid crystals. Our experiments show that ferroelectric nanoparticles reorient and hold liquid crystal molecules in a direction of the ferroelectric nanoparticles orientation even when an external electric field attempts to orient a liquid crystal in an orthogonal direction.

A High‐Density Azobenzene Side Chain Polymer Brush for Azimuthal and Zenithal Orientational Photoswitching of a Nematic Liquid Crystal

AbstractThe azimuthal and zenithal orientational switching of nematic liquid crystals (NLCs) can be performed by photoresponsive azobenzene alignment layers called command surfaces. Such photoresponsive layers have been prepared by molecular assembly methods or by spincasting of polymer materials. This work reports on the photoswitching orientation behavior of an NLC on a high‐density brush of an azobenzene‐containing side chain liquid crystalline polymer. On the high‐density polymer brush surface, the zenithal homeotropic/planar photoalignment switching modes triggered by UV/visible light irradiation are reversed from those observed for traditional command surface systems, reflecting the azobenzene side chain orientation directing parallel to the substrate surface. Because the thermally stable trans‐azobenzene provides the in‐plane orientation of the NLC, irradiation with linearly polarized visible light readily switches the azimuthal direction to an arbitrary direction, and the resulting orientation is retained persistently. This paper discusses the features of the architecture of a high‐density polymer brush as the command surface for NLC orientation.

Self-assembled vertical alignment layer of bifunctional amphiphiles for fast electro-optical switching of nematic liquid crystals

ABSTRACTWe present a novel homeotropic alignment method that simultaneously improves the ability of vertical alignment and fast switching of liquid crystals (LCs) in the electro-optical performance. The proposed method utilizes dual self-assembly of 4-(4-heptylphenyl)benzoic acid and 2-carboxyethyl acrylate via hydrogen bonding with ITO substrate. The 4-(4-heptylphenyl)benzoic acid composed of aromatic moieties and alkyl chains induce the homeotropic alignment of the LC molecules. Photopolymerization of 2-carboxylethyl acrylate with a reactive acrylate group under UV irradiation gives the LC molecules a fast response. Our proposed alignment method allows for fabrication of LC devices exhibiting fast LC switching without a conventional polyimide layer.

Superfast Low-Temperature Switching of Nematic Liquid Crystals Using Quasi-Impulsive Driving and Overdrive

In this paper, we report on the low-temperature, high-transmittance operation of a nematic liquid crystal (LC) cell. We investigated the electro-optic characteristics of a homogeneously-aligned LC cell over a wide temperature range. By applying high vertical and in-plane trigger pulse voltages between frames to an LC cell, the response time of the LC cell at $-{\hbox{20}} ^{\circ}{\hbox{C}}$ was decreased by 12.5 times compared to that of a conventional fringe-field switching (FFS) cell. In addition to providing a fast response, the LC cell exhibited the same high transmittance as an FFS cell over a wide temperature range.

Fast switching of nematic liquid crystals over a wide temperature range using a vertical bias electric field.

We propose a drive scheme using a three-terminal electrode structure for submillisecond switching of nematic liquid crystals (LCs). A vertical bias electric field is continuously applied to the LCs, whereas an in-plane electric field controls the switching to the bright state. Applying the proposed scheme to a homogeneously aligned nematic LC cell yields a submillisecond response time of 0.7ms at room temperature and 4.9ms at -20°C.

Technologies for Sub-millisecond Response Time of Nematic Liquid Crystals

In this paper, we introduce technologies for fast switching of nematic liquid crystals (LCs) using three-terminal electrodes. Fast switching of LCs can be achieved by employing a vertical trigger pulse, by applying a vertical bias field, and by random alignment of LCs.

Backflow-mediated domain switching in nematic liquid crystals.

We study the dynamics of the nematic liquid crystal kickback effect upon removal of a primary electric field and its amplification by a perpendicular secondary electric field resulting in the formation of domains with a reverse director orientation. Using computational fluid dynamics, we show that the domain formation is a robust phenomenon that takes place also in the complex case of multiple irregular random Freedericksz domains in three dimension as they appear in a realistic experimental situation. We propose domain switching by kickback amplification as a tool for self-insertion of shell-like inhomogeneities into an otherwise perfectly uniform director field configuration.

Submillisecond switching of nematic liquid crystal in cells fabricated by anisotropic phase-separation of liquid crystal and polymer mixture

Liquid crystal (LC) cells consisting of a very thin (submicrometer) nematic LC layer switch with time response smaller than 1ms. The total response time, i.e., sum of the turn-on and turn-off times, can be made as small as 1.3ms. Such devices were prepared by a photoinduced anisotropic phase separation of mixtures of the LC and prepolymer. The formation of the LC–polymer bilayer in these devices was confirmed by polarized light microscopy and scanning electron microscopy. The phase separation method permits one to fine-tune the LC film thickness by varying the LC concentration in the mixture. This technique can be used to fabricate fast LC devices for TV and video applications.

Bistable switching in chiral nematic liquid crystal layers with a 2π-twist ground state

We present experimental evidence and theoretical justification for the phenomenon of bistable electrooptical switching in nematic liquid crystals doped with chiral compounds so that the ground state corresponds to a 360° helical twist of the director. The system exhibits switching between the ground and a metastable unwound states. The observed effect differs from the well-known Berreman-Heffner bistable switching by the absence of a topologically nonequivalent ground state of the liquid crystal layer with a 180° twist of the director.

Modulated Microdomain Switching of Nematic Liquid Crystals

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.

In-plane switching electro-optical effect of nematic liquid crystals

This report describes the physical principles underlying the switching of nematic liquid crystals (LCs) by an electric field in the plane of the substrate. The results have contributed to development of an IPS-mode-TFT-LCD, which is now commercially available and one of the most popular TFT-LCDs with ultra-wide viewing angle characteristics. The report summarizes work in the Glenn Brown prize winning PhD thesis, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8551 Japan.

Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal

We have explored the change in alignment of a nematic liquid crystal, 4'-pentyl-4-cyanobiphenyl (5CB) with three types of photosensitive polyimide as the alignment layer by photoirradiation at 366 nm. The photosensitive polyimide alignment layer induced a reversible change in alignment of 5CB. It was observed that the 5CB molecules became aligned from homogeneous alignment to homeotropic on photoirradiation with a d.c. electric field as a bias, and reversed to the homogeneous state when photoirradiation was ceased. This result indicates that optical switching could be repeated by on and off switching of the excitation light at 366 nm. The optical switching of the nematic liquid crystal might be mainly due to a photophysical change in the polyimide surface which is affected by the chemical structures of the polyimides at the temperature at which 5CB exhibits a nematic phase. The optical switching of nematic liquid crystals with photosensitive polyimides as the alignment layer is a novel driving method for nematic liquid crystals.

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.

Optical switching of nematic liquid crystal by means of photoresponsive polyimides as an alignment layer

Photosensitive polyimides (PIs) as an alignment layer induced optical switching of nematic liquid crystal (NLC) on photoirradiation at 366 nm. The orientation of NLC molecule was changed from homogeneous to homeotropic alignment on photoirradiation with a dc electric field as a bias. The optical switching behavior of NLC was largely affected by the chemical structures of PIs.

1D03 Electro-Optical Switching of Nematic Liquid Crystals by Means of Polyimides Alignment Layer Having Different Photoactive Moieties

1D03 Electro-Optical Switching of Nematic Liquid Crystals by Means of Polyimides Alignment Layer Having Different Photoactive Moieties

Flexoelectrically controlled surface bistable switching in nematic liquid crystals

A new bistable surface switching device using nematic liquid crystals in bookshelflike geometry is presented. The surface bistability is controlled by an external pulsed electric field of defined polarity, which breaks the surface anchoring and selects the final state from dielectric and flexoelectric couplings. A writing time of 32 μs and an optical response time of 300 μs are achieved, for a field of 18 V across a 1 μm thick cell. This simple nematic optical device presents properties directly comparable with those of ferroelectric liquid crystals displays: bistability, sharp threshold, fast address time.

Flow induced bistable anchoring switching in nematic liquid crystals

Abstract A hydrodynamically controlled switching of orientations of a nematic bistable surface anchoring is observed. A viscous torque moves surface lines between domains corresponding to bistable anchoring. An observed threshold for this viscous torque is attributed to relative spatial inhomogeneities of surface energy (∼10−2).

Electrochirally controlled bistable surface switching in nematic liquid crystals

We present an electrically controlled bistable surface switching for nematic liquid crystals, in cells with the usual display geometry. The surface bistability is controlled by means of an electrochiral effect, by doping a nematic with chiral ions. The alternate transient commutation between the two surface states is related to the sign of an applied pulsed electric field. The threshold behavior allows a writing surface time of 32 μs for a field of 21 V/1.3 μm. In the birefringence mode, the optical texture response time is 0.3 ms. The effect, comparable with the one used with ferroelectric smectic liquid crystals, could be used to realize high definition video passive matrix displays.

Experimental and calculated results for the dynamics of oriented nematics with twist angles from 210° to 270°

A series of experimental data is presented on the switching of nematic liquid crystals with high twist, high tilt director configurations for step-voltage and pulse-voltage addressing. In order to describe the observed phenomena using a simple model containing a single viscosity coefficient only, we introduce an empirical correction to the rotational viscosity coefficient γ. The description appears adequate for step-voltage addressing but less so in the case of decay under specific pulse-voltage addressing conditions. Possible reasons are discussed.