LC Reorientation Research Articles

Ultralow‐Voltage Frequency Tunning of Liquid Crystal‐Based GHz Antenna via Nanoscratching Method

AbstractLiquid crystal (LC)‐based radio‐frequency (RF) tunable antennas garner attention because of their compact size and minimal heat generation. Exploiting the LC's property to alter the RF permittivity depending on its alignment, voltage‐induced orientational switching enables frequency modulation of the radiated wave. To ensure dependable antenna performance, precise control of the initial LC alignment is crucial, which is conventionally achieved by polyimide (PI) coating followed by the rubbing process (PI‐rubbing). However, this approach demands high temperature (200 °C) and prolonged processing time (2 h), while also necessitating high voltage for LC reorientation due to induced excessive surface polar anchoring strength. Here, a PI‐free‐nanoscratching method creating 1D channels is introduced that induces initial LC alignment in a desired direction. The initial orienting capability of the scratched metal surface is superior to that of the PI‐rubbed surface. Furthermore, the LC‐based antenna employing the nanoscratching method shows a remarkable resonance frequency shift of up to 3 GHz with a low voltage of 2 V. This driving voltage is five times lower than that of antennas using the conventional PI‐rubbing method, which can open a new era for the next generation LC‐antenna.

Enhancement of the physical parameters due to the dispersion of functionalised gold nanoparticles in a room temperature nematic liquid crystal

ABSTRACT A room temperature nematic liquid crystal (N-3017) mixed with surface-stabilised gold nanoparticles was examined for its thermodynamic, dielectric, and electro-optical properties. The isotropic to nematic transition temperature decreases in both the nanocomposites as compared to that of the pure nematic material, according to thermodynamic findings. In the frequency range of 1 Hz to 40 MHz, dielectric data of both the dispersed nanocomposites in homeotropic and planar aligned samples are collected. The dispersion of capped GNPs enhanced the dielectric anisotropy of the dispersed nanocomposites, according to the dielectric results. As the quantity of capped GNPs in the host matrix increases, the threshold voltage for LC reorientation decreases due to the decreased elastic constant and increased dielectric anisotropy. The dielectric study also signifies a prominent relaxation mode due to the flip-flop motion of the nematic molecules about their short axes. To analyse this mode, experimental data have been fitted with the modified Cole–Cole equation. This removes the parasitic effects in the low- and high-frequency domains.

Optical "Blinking" Triggered by Collisions of Single Supramolecular Assemblies of Amphiphilic Molecules with Interfaces of Liquid Crystals.

We report that incubation of aqueous dispersions of supramolecular assemblies formed by synthetic alkyl triazole-based amphiphiles against interfaces of thermotropic liquid crystals (LCs; 4-cyano-4'-pentylbiphenyl) triggers spatially localized (micrometer-scale) and transient (subsecond) flashes of light to be transmitted through the LC. Analysis of the spatiotemporal response of the LC supports our proposal that each optical "blinking" event results from collision of a single supramolecular assembly with the LC interface. Particle tracking at the LC interface confirmed that collision and subsequent spreading of amphiphiles at the interface generates a surface pressure-driven interfacial flow (Marangoni flow) that causes transient reorientation of LC and generation of a bright optical flash between crossed polarizers. We also found that dispersions of phospholipid vesicles cause "blinks". When using vesicles formed from 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), we measured the frequency of blinking to decrease proportionally with the number density of vesicles in the aqueous phase, consistent with single vesicle events, with the lifetime of each blink dependent on vesicle size (800 ± 80 nm to 150 ± 30 nm). For 100 μM of DLPC, we measured vesicles with a diameter of 940 ± 290 nm to generate 47 ± 9 blinks min-1 mm-2, revealing that the fraction of vesicle collisions resulting in fusion with the LC interface is ∼10-3. Overall, the results in this paper unmask new nonequilibrium behaviors of amphiphiles at LC interfaces, and provide fresh approaches for exploring the dynamic interactions of supramolecular assemblies of amphiphiles with fluid interfaces at the single-event level.

The interaction of polymer dispersed liquid crystal sensors with ultrasound

Polymer dispersed liquid crystals (PDLCs) have been shown to be sensitive to ultrasound through the acousto-optic effect. The acousto-optic response of PDLCs was studied over a broad frequency range (0.3–10 MHz). We demonstrate that the displacements required to produce acousto-optic clearing of PDLC films can be as low as a few nanometers, which is at least 103 times smaller than the PDLC droplet size, is 105 times smaller than the PDLC layer thickness, and of the order of the molecular size of the liquid crystal constituents. This suggests that the acousto-optic effect in PDLCs is due to the microscopic effects of the LC reorientation under torques or flows rather than the LC reorientation through macroscopic droplet deformation. The displacement required for clearing is related to the frequency of operation via an exponential decay. We attribute the observed frequency response to a freezing out of the rotational motion around the short axis of the liquid crystal. The reported frequency dependence and displacements required indicate that the effects and materials described here could be used for ultrasound visualization in a non-destructive testing context.

Optomechanics in anisotropic liquid crystal -filled micro-bubble resonators.

In this paper, we report on the realization of optomechanics and its broad tuning in liquid crystal-filled micro-bubble resonators (LC-MBR). Acoustic anisotropy of LC has enriched optomechanical resonant modes in such whispering-gallery modes (WGM) cavities. Meanwhile anisotropic dependence of sound speed of LC on temperature strongly enhances the tuning ability of the resonances. By applying magnetic field to control re-orientation of LC in MBR, optomechanical resonant frequency changes significantly after magnetic field goes beyond Fréedericksz transition threshold. By designing such an optomachanical system, optomechanical oscillator with broad tuning range can be realized.

Electro-optical Effect of Gold Nanoparticle Dispersed in Nematic Liquid Crystals

The electro-optical performance of nematic liquid crystals (NLCs) dispersed with gold nanoparticles (GNPs) was investigated in this study. The addition of a small amount of GNP dopant decreased the threshold voltage for LC reorientation due to the decreased elastic constant and increased dielectric anisotropy of the LC mixture. The response time of the LC cell was decreased with the addition of tiny amounts of GNPs because of decrease in rotational viscosity of LCs. The doped GNPs also shorten LC reorientation angle during voltage switching, further decreasing the response time of the LC cell. The addition of high amounts of GNPs slowed down the response time of the LC cell, because excess GNPs aggregated and formed networks in the cell, thus disturbing LC alignment and hindering LC reorientation. The measured dielectric spectra of the GNP-LC mixture revealed that the addition of GNPs decreased the relaxation time constant of the LCs. This result confirmed that the GNP dopant decreased the rotational viscosity and elastic constant of the LCs.

Red light holographic recording and readout on an azobenzene–LC polymer hybrid composite system

We present for the first time stable red light two-dimensional image holographic recording in azobenzene-containing polymer-liquid crystal material following a pre-illumination process. The red light hologram recording is based on cis-to-trans transition of the azobenzene and surrounding LC reorientation. The diffraction efficiency of about 2% has been achieved and clear holograms of a commercial 2D USAF resolution target have been recorded. Using the same readout wavelength as the recording wavelength, color distortion that exists in most hologram researches with azo-containing materials is naturally eliminated. Long-time stabilities such as one-day continuous non-destructive readout and up to 10 weeks hologram storage have been demonstrated. The holographic recording material remains erasable and rewritable.

Liquid-crystal-tunable metal–insulator–metal plasmonic waveguides and Bragg resonators

The dispersive properties of liquid-crystal-tunable metal–insulator–metal plasmonic waveguides are theoretically investigated as a function of the applied control voltage. The LC reorientation study is rigorously coupled to the optical studies via a multiphysics finite-element analysis. A Bragg grating resonator is designed based on the proposed waveguide, which shows an extensive tuning efficiency of more than 100 nm V−1 for low operating voltages up to 2 V.

Liquid Crystal Reorientation Induced by Aptamer Conformational Changes

Aptamer-ligand binding events, involving small molecule targets, at a surfactant-laden aqueous/liquid crystal (LC) interface were found to trigger a LC reorientation that can be observed in real-time using polarized light. The response was both sensitive and selective: reorientation was observed at target concentrations on the order of the aptamer dissociation constant, but no response was observed in control experiments with target analogues. Circular dichroism and resonance energy transfer experiments suggested that the LC reorientation was due to a conformational change of the aptamer upon target binding. Specifically, under conditions where aptamer-ligand binding induced a conformational change from a relaxed random coil to more intricate secondary structures (e.g., double helix, G-quadruplex), a transition from planar to homeotropic LC orientation was observed. These observations suggest the potential for a label-free LC-based detection system that can simultaneously respond to the presence of both small molecules and nucleic acids.

Jump-Like Light-Induced Hydrodynamic Reorientation of LC's Caused by Direct Volume Expansion

The dynamics of the direct volume expansion mechanism of light induced hydrodynamic motions in homeotropic and hybrid oriented NLC have been studied theoretically. In case of homeotropic initial orientation of NLC molecules the velocity gradient induces a curvature of director. In the case of hybrid initial orientation of NLC velocity gradient brings about a small increase of curvature when hydrodynamic flow velocity is directed out of the “flexible ribbon's” curvature. The “flexible ribbon” reverses its curvature when velocity is directed into “flexible ribbon's” curvature.

Transient Patterns in the Magnetic Reorientation of Low Molecular Mass Nematic Liquid Crystals

Abstract We present the results of optical investigations of transient patterns appearing during the reorientation of nematic LC in magnetic fields. The transition from a planar modulated director field to a metastable non-planar structure of twist-bend walls is observed. From the wavelengths and characteristic rise times of the investigated patterns, the validity of the linearized mode selection theory is confirmed and viscoelastic parameters are determined.

Characteristics of liquid crystal dispersed in polymer matrix

Abstract A PDLC film composed of PMMA and E72 shows characteristic changes in transmittance with changes of temperature. The film responds fast to the applied voltage in orientation of LC. However, the response measured as transmittance after switching-off the applied voltage reveals the distinctive information on LC mobility-temperature relationship. The reorientation of LC after switching-off the voltage takes the longest time at 38°C. The temperature coincides with Tg measured by DSC. The slowest response in terms of transmittance at 36°C is attributed to the resonance developed between the polymer segments wobbling at Tg and LC returning to the state prior to switching-on.

Optically induced hydrodynamic reorientation of liquid crystals and its applications for infrared detection and information storage.

Absorption mediated effects provide the strongest mechanisms of optically induced reorientation of liquid crystals (LC's). The following mechanism of transformation of absorbed light energy into reorientation of LC's is theoretically discussed in the present paper: thermal expansion resulting from absorption induced heating causes inhomogeneous flow of the LC in the capillary which, due to the strong coupling of hydrodynamic and orientational degrees of freedom, leads to LC reorientation. With a plane capillary, this may allow detection of infrared radiation at a microwatt level. With microcapillaries in porous media, this may lead to chaotization of orientation and creation of scattering pixels allowing for high resolution optical information storage matrixes. Estimations done for so called ``filled nematic liquid crystals'' are in good accordance with existing experimental data.