Electro-optical Behavior Research Articles

Optical and electro-optic anisotropy of epitaxial PZT thin films

Strong optical and electro-optic (EO) anisotropy has been investigated in ferroelectric Pb(Zr0.48Ti0.52)O3 thin films epitaxially grown on Nb-SrTiO3 (001), (011), and (111) substrates using magnetron sputtering. The refractive index, electro-optic, and ferroelectric properties of the samples demonstrate the significant dependence on the growth orientation. The linear electro-optic coefficients of the (001), (011), and (111)-oriented PZT thin films were 270.8, 198.8, and 125.7 pm/V, respectively. Such remarkable anisotropic EO behaviors have been explained according to the structure correlation between the orientation dependent distribution, spontaneous polarization, epitaxial strain, and domain pattern.

Investigations on electro-optical and thermal performances degradation of high power density GaAs-based laser diode in vacuum environment

Pump laser diodes emitting at 980nm are typically used in distribution network transmission systems and are now implemented in space missions for intra-satellite communication links and calibration systems. Nevertheless, since the pump source is potentially a critical point for such systems, there is a huge need to assess their reliability in space environment, especially in terms of operation under vacuum conditions. We have then studied the electro-optical behavior of several 980nm laser diodes and shown that they may degrade more rapidly when submitted to a vacuum environment. Then, spatially resolved temperature variations have been measured by thermoreflectance on the laser diode front facet and have shown a temperature variation increase under vacuum conditions, in particular near the emission region, where a catastrophic optical damage is most likely to occur. This temperature variation increase could then be one of the causes for the more rapid laser diode degradation.

Screen printing insulator coatings for electrofluidic display devices

The screen printing of amorphous fluoropolymer films as insulators for electrofluidic display device use is demonstrated. Screen printing enables the hydrophobic dielectric to be directly patterned at device level with the required film thickness as well as excellent material utilisation. The films give very good electrical performance, with high uniformity and dielectric strength, and the resulting devices exhibit comparable electro-optic behaviour to those fabricated with spin-coated films.

Understanding the unusual reorganization of the nanostructure of a dark conglomerate phase.

The dark conglomerate (DC) phase exhibited by a bent-core liquid crystal shows remarkable properties including an electric-field tunable chiral domain structure and a large (0.045) reduction of refractive index, while maintaining an optically dark texture when observed under crossed polarizers. A detailed investigation of the system is presented, leading to a model that is fully consistent with the experimental observations. It reports the observation of two distinct regimes in the DC phase: a higher temperature regime in which the periodicity measured by small angle x-ray scattering decreases slightly (0.5%) and a lower temperature regime where it increases considerably (16%). Also, the paper discusses the unusual electric-field-induced transformations observed in both the regimes. These changes have threshold fields that are both temperature and frequency dependent, though the phenomena are observed irrespective of device thickness, geometry, and the alignment layer. The electro-optic behavior in the DC phase corresponds to a number of structural changes leading to unusual changes in physical properties including a small (1%) increase in periodicity and a doubling of the average dielectric permittivity. We propose a model of the DC phase where in the ground state the nanostructure of the phase exhibits an anticlinic antiferroelectric organization. Under an electric field, it undergoes a molecular rearrangement without any gross structural changes leading to an anticlinic ferroelectric order while keeping the overall sponge-like structure of the DC phase intact.

Improving the stability of organosiloxane smectic A liquid crystal random lasers using redox dopants

In this paper, we focus on the development of liquid crystal (LC) visible-light scattering devices for random lasers. These light-scattering devices are based upon binary mixtures that consist of an organosiloxane smectic A LC and a wide temperature range nematogen LC. Both the temperature range of the smectic A phase and the dielectric anisotropy of the binary mixture are increased compared with that of the neat organosiloxane compound. In the latter case, the increase in the dielectric anisotropy results in a reduction of the magnitude of the electric field required to induce a clear state. Furthermore, it is found that the electric field threshold continues to decrease with increasing concentration of the nematic compound. For the random laser devices, the Pyrromethene 597 laser dye was added to a mixture that was optimized for scattering and it was found that the absorption properties of the dye becomes unstable in the presence of the electro-hydrodynamic instabilities that are required to generate scattering in the LC cells. This is believed to be due to electro-chemical reactions that occur at the electrodes. To avoid dye degradation and ensure repeatable electro-optic behaviour, a reduction–oxidation (redox) couple is dispersed within the dye-doped binary mixture. It is shown that the addition of redox dopants helps to stabilize the dye in the scattering mixtures, and also increases the long-term repeatability of the scattering behaviour. Finally, we conclude by characterizing the random laser emission of the dye-doped binary mixture and demonstrate improved stability.

Numerical and experimental investigations of micro-integrated external cavity diode lasers

We report the results of numerical and experimental investigations of the electro-optical behaviour of an external cavity diode laser device composed of a semiconductor laser and a distant Bragg grating, which provides an optical feedback. The travelling wave model was used to simulate and analyse the nonlinear dynamics of the considered laser device. Different modelling approaches for gain and refractive index change functions have been considered. It is shown, that the simulated behaviour of the electro-optical characteristics is in good agreement with experiments.

Topology-mediated electro-optical behaviour of a wide-temperature liquid crystalline amorphous blue phase.

A wide-temperature liquid crystalline amorphous blue phase based on polymer network stabilization exhibits high thermodynamic stability with an extended temperature ranging from a few °C to more than 80 °C (including room temperature). Analyses using confocal laser scanning microscopy show that the polymer network imitates the aperiodic disclination-entangled structure of an amorphous blue phase and involves a highly intertwined interaction with liquid crystal molecules. The stabilized amorphous blue phase manifests a tens of microseconds response time, a consistent achromatic dark state and it is intrinsically hysteresis-free during the application of an electric field. The topological and electro-optical features of the stabilized amorphous blue phase are further compared with the stabilized isotropic and cubic blue phases. These results not only provide a physical perspective on the electro-optical response of a liquid crystal and polymer composite but also open up a new direction for electro-optical device applications.

The relationship between molecular structure and the incidence of the NTB phase

In this work, we present the first part of a study into the relationship between molecular structure and the occurrence of the ‘twist-bend nematic phase’ (NTB). Given the large amount of chemical space that might reasonably be expected to give rise to the NTB phase, this paper is only concerned with methylene-linked bimesogens bearing polar terminal groups based on the initial work of George Gray on cyanobiphenyls. As with other studies, we find that the NTB phase is observed only for materials that contain an odd number of methylene units in the spacer chain. It also appears that, in a given series of materials, there is a weak negative correlation between the dipole moment of the individual mesogenic units and the thermal stability of the NTB phase. Furthermore, we find that increasing the length–breadth ratio of the individual mesogenic units also provides a significant increase in the thermal stability of the NTB phase. The electrooptic behaviour of two materials, one with a terminal nitrile unit and one with an isothiocyanate group, was investigated. The NTB phase of the NCS-terminated material can be switched with a large applied voltage (20 V μm−1); however, the analogous nitrile-terminated material showed no electrooptic response under these conditions. Either the threshold voltage to switching is simply lower for isothiocyanate materials than nitriles or that there is more than one phase currently identified as the twist-bend nematic.

A Reflective Display Technology Based on Electrofluidics

In this paper we describe the design, driving and electro-optical behavior of an electrofluidic, or electrowetting, display panel. Electrofluidic displays involve the movement of a colored oil water interface as a result of polarizing the interface between water and a hydrophobic surface under the influence of external electric field. The proposed direct-segment drive approach successfully drives the display panel based on electrofluidic. The electro-optical behavior of electrofluidic display samples are measured and analyzed. The tonand toffof the pixel is 16 ms and 8 ms, respectively, which demonstrates that electrofluidic display panels have the capability to show video content.

Tri-Stable Polarization Switching of Fluorescent Light Using Photo-Luminescent Cholesteric Liquid Crystals

Using a photo-luminescent cholesteric liquid crystal (PL-CLC) cell, tri-stable electro-optic switching behaviour was demonstrated. Stable planar, uniformly lying helix (ULH), and focal conic states were obtained at an electric field of zero after applying proper electric signals corresponding to each state. When a PL-CLC cell was placed on a ultraviolet (UV) back-light, the cell emitted fluorescent light with different types of polarization corresponding to the liquid crystal (LC) state: the planar, ULH, and focal conic states emitted circularly polarized light, linearly polarized light, and un-polarized light, respectively. Hence, the proposed PL-CLC cell worked as a tri-stable polarization-switching light source. We also demonstrated that the PL-CLC cell can be used to fabricate a bistable fluorescent LC device.

Strain-induced long range ferroelectric order and linear electro-optic effect in epitaxial relaxor thin films

Relaxor ferroelectrics have neither long range ferroelectric order nor structural transformation down to the lowest temperatures, and display isotropic optical properties like quadratic electro-optic effect. However, if an anisotropy is forced through an external agent, like electric field or uniaxial strain, a ferroelectric and structural long range order can be induced in these materials. Here, we show that epitaxial strain in relaxor ferroelectric thin films can be employed to induce a linear electro-optic effect, opening the path to new strain-controlled electro-optic materials. Epitaxial thin films with Pb1-3x/2LaxZr0.2Ti0.8O3 (x = 0.22) composition grown by pulsed laser deposition on (001) SrRuO3/SrTiO3 single crystal heterostructures become tetragonal below the susceptibility peak, which occurs at a temperature 140 K higher as compared to bulk. These films show piezoelectric properties and almost linear electro-optic behaviour.

Investigations of the electro-optic behavior of Cs2TeMo3O12 single crystal

The electro-optic (EO) property of hexagonal Cs2TeMo3O12 (CTM) crystal is studied with interference technique at 632.8 nm. Both the value and sign of the EO coefficients have been obtained and confirmed. The largest EO coefficient is 10.4 pm/V for r33 due to its structure distortion along c-axis. The transverse effective EO effect of r33 and the half-wave voltage are calculated to be 11.08 pm/V and 5200 V at 1064 nm, respectively. The experimental results show that CTM can be one of promising candidates for the EO applications, especially in the mid-infrared region.

Homeotropic orientation of nematic liquid crystals induced by dissolving polypropyleneimine dendrimer having peripheral mesogens

A mixture of nematic liquid crystals (NLCs) with a small amount of a polypropyleneimine-based liquid crystalline dendrimer (D-6PC5 and D-6PPCN) exhibited spontaneous homeotropic alignment between the native glass substrates. This dendrimer-induced homeotropic alignment required two conditions; 1) the dendrimer completely dissolves in the NLC, and 2) a substrate surface is hydrophilic with the surface free energy above approximately 65 mN·m−1. The interdigitated-electrode cells without any surface treatment of the substrates were fabricated by filling with the LC dendrimer (D-6PC5)/nematic LC mixtures. They exhibited an electro-optical behavior when applying AC, vertical-alignment drive occurred in the cells. Based on these results, we speculated that the dendrimer adsorbs on the substrate surface and acts as a vertical alignment layer.

Third column electro-optical coefficients of zirconium-doped congruent lithium niobate crystals

All coefficients of the third-column of the unclamped and clamped electro-optic tensor of zirconium-doped congruent LiNbO3 single crystals, in the concentration range up to 2.5mol%, are determined as function of the dopant concentration by direct techniques based on interferometric and Sénarmont optical arrangements at the wavelength of 633nm and at room temperature. It is found that all the unclamped and clamped electro-optic coefficients are relatively constant, except for the sample doped with 2mol% of zirconium. From the obtained experimental values, the acoustic contribution is then deduced and found constant over all the studied samples. Finally, the electro-optic behavior of LN:Zr as function of the dopant concentration was confirmed by dielectric characterizations.

The complete morphological, electro-optical and dielectric study of dichroic dye-doped polymer-dispersed liquid crystal

Polymerisation-induced phase separation method was used to prepare dye-doped polymer-dispersed liquid crystal (DPDLC) films consisting of nematic liquid crystal, photo-curable pre-polymer and a dichroic dye. The change occurring on droplets morphology with the change in dye concentration and voltage was investigated by means of polarising optical microscope and scanning electron microscope. Electro-optic behaviour of these composite films in the presence of an externally applied AC electric field (0–100 V, 200 Hz) and temperature range 25–50°C was evaluated using He–Ne laser. The maximum absorbance of dye and effect of electric field on the absorbance of dye was studied using UV-Vis spectrophotometer. Dielectric relaxation spectroscopy was carried out in the frequency range 20 Hz–20 MHz and over the temperature range 25°C–90°C. Dielectric parameters such as distribution parameter, relaxation frequency, relaxation strength and relaxation time of these DPDLC films were calculated, and the modelling of experimental data was done using Debye and Cole–Cole method.

Electro-Optical Study of Fluorescent Dye-Doped Ferroelectric Liquid Crystal

The present paper reports the electro-optical behavior of a pure and fluorescent dye-doped ferroelectric liquid crystal (FLC) system. Different material parameters, such as spontaneous polarization and optical response time, have been analyzed by electro-optical study. The temperature dependence of these parameters have been discussed for the pure and the fluorescent dye-doped systems. The spontaneous polarization and optical response time have also been found to change strongly by the variation of concentration of the fluorescent dye into the pure FLC. In addition to these parameters, the present paper also reports the temperature dependence of the effective dielectric permittivity for the pure and fluorescent dye-doped FLC systems. The value of spontaneous polarization has improved after doping of fluorescent dye suggesting that fluorescent dye is beneficial for this FLC material.

Synthesis of Liquid Crystal Silane‐Functionalized Gold Nanoparticles and Their Effects on the Optical and Electro‐Optic Properties of a Structurally Related Nematic Liquid Crystal

Chemically and thermally robust liquid crystal silane-functionalized gold nanoparticles (i.e. AuNP1-AuNP3) were synthesized through silane conjugation. Colloidal dispersions of these particles with mesogenic ligands that are structurally identical (as in AuNP1, AuNP2) or compatible (as in AuNP3) with molecules of the nematic liquid crystal (N-LC) host showed superior colloidal stability and dispersibility. The thermal, optical, and electro-optic behaviors of the N-LC composites at different concentrations of each gold nanoparticle were investigated. All dispersions showed lower values for the rotational viscosity and elastic constant, but only AuNP3 with a dissimilar structure between the nanoparticle ligand and the host displayed the most drastic thermal effects and overall strongest impact on the electro-optic properties of the host. The observed results were explained considering both the structure and the density of the surface ligands of each gold nanoparticle.

Propagation through complex structured liquid crystal optical fibers

Wave propagation in optical mediums greatly depends on the materials of which the guides are composed. Among the other forms of optical mediums, liquid crystals are both inhomogeneous and optically anisotropic in nature, and exhibit strong electro-optic behavior, which allows alternation in their optical properties under the influence of external electrical fields. These features make optical fibers containing liquid crystals greatly useful for fabricating many optical devices for practical applications. As such, the analytical investigation of wave propagation through liquid crystal optical fibers, particularly a three-layer fiber with radially anisotropic liquid crystal material in the outermost clad section, remains interesting. The power confinement in the liquid crystal section of such fibers can be enhanced for these to be efficiently used in optical coupling and/or sensing applications. Furthermore, a control over the dispersion characteristics, and, hence, the confinement of power, in such fibers may be imposed by making the guide even more complex in the form of introducing a conducting sheath helix structure at the core-inner clad interface.

Electro-optic and molecular relaxation behaviour of fluoro substituted achiral unsymmetrical four-ring bent-core mesogen

Electro-optic and molecular relaxation studies in a new achiral unsymmetrical four-ring bent-core mesogen are presented in this study. The compound consists of a laterally fluoro-substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the two phenyl rings as the central unit. The mesomorphic properties were characterised by polarising optical microscopy and differential scanning calorimetry. The compound exhibited enantiotropic B7–Bx phase sequence. The electro-optic studies revealed a bistable switching from planar to a homeotropic state under an electric field in B7 phase. Further memory effect was observed between planar and homeotropic textures after the removal of electric field in B7 phase. Temperature dependent permittivity studies as a function of frequency revealed that the molecular relaxation increases with the increasing temperature in B7 phase than in Bx phase.

Electro-optical behaviour and dielectric dynamics of harvested ferroelectric LiNbO3 nanoparticle-doped ferroelectric liquid crystal nanocolloids

Nanocolloids comprising harvested ferroelectric LiNbO3 nanoparticles (≈25 nm) and ferroelectric liquid crystals were prepared and investigated. The impact of the amount of dispersed nanoparticles (0.01 wt%, 0.05 wt%, 0.10 wt%) on the electro-optical, dielectric and material parameters of the ferroelectric liquid crystals (FLCs) has been demonstrated. The dopant strongly influences the switching characteristics and has an effect on the saturation voltage of the FLC materials. Electro-optical measurements demonstrated that a relatively large (0.10 wt%) LiNbO3 dopant concentration decreases the response time and saturation voltage by approximately 40% and 25% respectively, compared with non-doped FLCs. The effect of LiNbO3 nanoparticle concentration on the material and dynamical parameters (i.e. spontaneous polarization, dielectric permittivity, dielectric absorption, dielectric strength, dc conductivity, and rotational viscosity) is presented. The observed behaviours are discussed in terms of capture of impurity ions on the surface of the harvested LiNbO3 nanoparticles. Ion capturing leads to enhancement of the localized electric field inside the smectic layers. The fast switching response and the low operational voltage of these FLC nanocolloids support their application in nanometric scale electronic devices.