Electric Field-induced Birefringence Research Articles

Molecular orientational distribution function of a chiral de Vries smectic liquid crystal from birefringence measurements.

An alternative method for determining the orientational distribution function and the order parameter from the electric field-induced birefringence measurements of a chiral liquid crystal compound in its Smectic A* is being introduced. A chiral mesogen based on a 5-phenyl-pyrimidine benzoate core terminated by a trisiloxane group on one side and the chiral alkyloxy chain on its opposite side is designed and synthesized to exhibit the "de Vries" smectic characteristics. The compound exhibits first order Smectic A*-Smectic C* phase transition, evidenced by the results of differential scanning calorimetry. The material is being investigated by electro-optical experiment in its smectic phases. We present a model that incorporates the generalised Langevin-Debye model which includes the Maier-Saupe effective mean-field potential term in order to explain the change in birefringence with the electric field. A good agreement between the experimental results and the predictions from the model leads to the determination of the molecular orientational distribution function in Smectic A phase. Furthermore, the temperature dependency of the Saupe orientational order parameter ⟨P2⟩ is obtained using the parameters of the model. Based on the experimental and theoretical results, we show that de Vries Smectic A* phase exhibits a broad volcano-like tilt angle distribution with the two maxima occurring at finite tilt angles closer to the Smectic A*-Smectic C* transition temperature, and a sugarloaf-like distribution occurs in the tilt for temperatures close to the Isotropic-Smectic A* phase transition.

Ferrielectric six-layer () and several electric-field-induced subphases in AS657 studied by complementary methods, electric-field-induced birefringence and microbeam resonant X-ray scattering

ABSTRACTA selenium-containing compound, AS657, is studied with two complementary methods, electric-field-induced birefringence (EFIB) and microbeam resonant X-ray scattering (µRXS). AS657 shows a phase sequence of Sm – – – Sm – Sm where no Sm emerges and hence the frustration between Sm and Sm plays an important role. In spite of this situation, two additional temperature-induced subphases are observed: (1) antiferroelectric eight-layer between Sm and and (2) antiferroelectric 10-layer between and . A number of field-induced subphases are also shown to emerge. The most notable new finding is the observation of an electric-field-induced ferrielectric subphase of six-layer () between and Sm; this subphase is stabilised even at zero field in a mixture of 92.6 wt% AS657 and 7.4 wt% AS620. Another field-induced subphase of six-layer is also confirmed to emerge in the temperature ranges of SmCA* and . The nature of the field-induced transitions in the temperature range is more complex than previously thought; in its high temperature range, wound (not unwound) Sm with some macroscopic distorted helical director arrangement reappears above , whereas in the low temperature range, some intriguing sequential field-induced transitions are observed both in the EFIB and µRXS. Last but not the least is the observation of a field-induced subphase found in the low-temperature range of by EFIB and this is tentatively assigned as , the detailed structure of which is expected to be clarified by µRXS in the near future.

Definite existence of subphases with eight- and ten-layer unit cells as studied by complementary methods, electric-field-induced birefringence and microbeam resonant x-ray scattering.

A mixture of two selenium-containing compounds, 80 wt.% AS657 and 20 wt.% AS620, are studied with two complementary methods, electric-field-induced birefringence (EFIB) and microbeam resonant x-ray scattering (μRXS). The mixture shows the typical phase sequence of Sm-C_{A}^{*}-1/3-1/2-Sm-C^{*}-Sm-C_{α}^{*}-Sm-A, where 1/3 and 1/2 are two prototypal ferrielectric and antiferroelectric subphases with three- and four-layer unit cells, respectively. Here we designate the subphase as its q_{T} number defined by the ratio of [F]/([F]+[A]), where [F] and [A] are the numbers of synclinic ferroelectric and anticlinic antiferroelectric orderings in the unit cell, respectively. The electric field vs temperature phase diagram with EFIB contours indicates the emergence of three additional subphases, an antiferroelectric one between Sm-C_{A}^{*} and 1/3 and antiferroelectric and apparently ferrielectric ones between 1/3 and 1/2. The simplest probable q_{T}'s for these additional subphases are 1/4,2/5, and 3/7, respectively, in the order of increasing temperature. The μRXS profiles indicate that antiferroelectric 1/4 and 2/5 approximately have the eight-layer (FAAAFAAA) and ten-layer (FAFAAFAFAA) Ising unit cells, respectively. The remaining subphase may be ferrielectric 3/7 with a seven-layer unit cell, although the evidence is partial. These experimental results are compared with the phenomenological Landau model [P. V. Dolganov and E. I. Kats, Liq.Cryst. Rev. 1, 127 (2014)2168-039610.1080/21680396.2013.869667] and the quasimolecular model [A. V. Emelyanenko and M. A. Osipov, Phys. Rev. E 68, 051703 (2003)1063-651X10.1103/PhysRevE.68.051703].

Thermal Behavior of Electrically Induced Birefringence of La-Doped PMN-PT Ceramics

The thermal evolution of the electric field-induced birefringence, δ(Δn(E)) vs T, obtained by the Kerr and Pockels electro-optical effects, was characterized in the La-doped Pb(Mg1/3Nb2/3)O3 – PbTiO3 ferroelectric relaxor ceramic system. The measurements were conducted over a broad temperature range (from 100 K to 650 K), using the dynamic Senarmont compensator method. Moreover, through a particular approach to the analysis of both electro-optical effects as a function of temperature, it was possible to determine the characteristic temperatures predicted for the transitions between the paraelectric (PE), ergodic ferroelectric relaxor (EFR), nonergodic ferroelectric relaxor (NEFR), and normal ferroelectric (FE) states of this system.

Discovery of a novel ferrielectric phase of five-layer periodicity in binary mixtures of chiral smectic liquid crystals exhibiting unusual reversed phase sequence

In a binary mixture system of ferroelectric and antiferroelectric liquid crystals whose major component shows an unusual reversed phase sequence of , a new phase with ferrielectric order of five layers has been discovered by the electric field-induced birefringence (EFIB) measurements. The EFIB was measured using a photo-elastic modulator (PEM) set-up and by applying an in-plane electric field to a homeotropic aligned cell filled with the binary mixtures of compounds with ferroelectric and antiferroelectric compounds. The contours of constant birefringence in the electric field–temperature (E–T) phase diagrams clearly indicate a distinct region corresponding to a new phase bordering the four-layer on the low temperature side and on the high temperature side. This new phase is unambiguously assigned to whose structure has been calculated by Osipov and Gorkunov.

Electrooptic response of liquid crystalline blue phases with different chiral pitches

The relationship between the electrooptic response properties and the chiral pitch of a blue phase was experimentally investigated. The Kerr constant was found to be proportional to the cube of the chiral pitch, although a theoretical square-law was expected. It was also revealed that the effect of the chiral pitch on the response time was dependent on the type of electric field-induced birefringence of the blue phase, such as local director reorientation and electrostriction. The response of the local director reorientation process was very fast at sub-millisecond and also proportional to the cube of the chiral pitch. However, in the case of the electrostriction process, the effect of the chiral pitch on the response time was insignificant and the response time was a few milliseconds.

Polymer-stabilized optically isotropic liquid crystals for next-generation display and photonics applications

Polymer-stabilized optically isotropic liquid crystals, including blue phases, are emerging as a strong contender for next-generation display technology because they exhibit some revolutionary features such as no need for surface alignment, submillisecond response time, isotropic dark state, and cell gap insensitivity. The basic material properties, including electric field-induced birefringence, dispersion relation of Kerr constant, and temperature dependent Kerr constant, are reviewed. Recent progress on blue phase liquid crystal material development and device structures for lowering the operating voltage are introduced. Promising applications and remaining technical challenges are also discussed.

Tunable optical properties of Au nanofluids under electric field

Using a repeating deoxidization method, we prepared Au nanoparticles with different mean sizes of 11 nm, 35 nm and 58 nm. Then the Au nanoparticles were dispersed in mineral oil to form Au nanofluids by the surfactant-assisted phase transition technique. Under electric field, the optical properties of the Au nanofluids were measured. It was found that the Au nanofluids exhibit significant electric field-induced birefringence and the birefringence index can be adjusted by changing the external electric field strength. Furthermore, the birefringence index is influenced by the Au concentration and particle size. Within the suitable particle concentration range, the index decreases with the increase of Au concentration and particle size. Finally, we further discuss the electric field-induced birefringence phenomena of the Au nanofluids according to the structure transformation mechanism of electrorheological fluids under electric field.

62.2: Invited Paper: Fast Electro‐Optical Switching in Polymer‐Stabilized Liquid Crystalline Blue Phases for Display Application

AbstractBlue phases have two major advantages over commonly used nematic phases:1) the response is much faster, 2) the zero‐electric field state is optically isotropic. We demonstrate the sufficiently large electric field‐induced birefringence and the micro‐second response of the polymer‐stabilized blue phases and the induced‐isotropic phases without any surface treatment.

Generalized ellipsometry in unusual configurations

Most ellipsometry experiments are performed by shining polarized light onto a sample at a large angle of incidence, and the results are interpreted in terms of thin film thicknesses and isotropic optical functions of the film or substrate. However, it is possible to alter the geometrical arrangement, either by observing the sample in transmission or at normal-incidence reflection. In both cases, the experiment is fundamentally the same, but the interpretation of the results is considerably different. Both configurations can be used in conjunction with microscope optics, allowing for images to be made of the sample. The results of three examples of these different configurations using the two-modulator generalized ellipsometer (2-MGE) are reported: (1) spectroscopic birefringence measurements of ZnO, (2) electric field-induced birefringence (Pockels effect) in GaAs, and (3) normal-incidence reflection anisotropy of highly oriented pyrolytic graphite (HOPG).

Temperature Dependence of Photorefractive Properties of PVK-based Composites

We studied the temperature dependencies of electric field-induced birefringence, conductivity, and photorefractive properties of two poly(n-vinylcarbazole) (PVK)-based composites with different nonlinear optical chromophores: 2,5-dimethyl-4-(4'-nitrophenylazo)anisole (DMNPAA) and its derivative, 4-butoxy-3-propyl-1-(4'-nitrophenyazo) benzene (m3p4). Both composites showed similar temperature dependencies; the photorefractive responses were faster at higher temperatures and the diffraction efficiencies increased with temperature and then began to decrease. Such behavior is consistent with the measured temperature dependencies of the field-induced birefringence and the conductivity. We found that the m3p4 composite exhibited a faster photorefractive response rate than the DMNPAA composite when compared at the same reduced temperature T-Tg, where T is the temperature of the measurement and Tg is the glass transition temperature. The results indicate that one can enhance the total photorefractive response rate by changing the nonlinear optical chromophore.

Modeling of the Electric Field-Induced Birefringence of Vesicles

Electric birefringence of unilamellar lipid vesicles, elongated in the direction of the applied field E, is modeled for a low degree of deformation to a prolate ellipsoid of the original spherical shells. The existing theory for water-in-oil microemulsion is adapted, and the intrinsic birefringence of the lipid membrane and the form birefringence of the aqueous inner compartment of the vesicle are taken into account. Deformation under both constant surface area and constant volume conditions are considered, but thinning of the bilayer at the polar caps and rotational/orientational effects are neglected. For illustration of the utility of the theoretical results, the extent of elongation of large unilamellar vesicles (LUV), prepared from the zwitterionic DOPC (dioleoylphosphatidylcholine), is calculated based on available experimentally measured induced birefringence data. With the use of reasonable values for the refractive indexes of the membrane, the axial ratio P of the deformed vesicle is found to be ...

Electric Field-Induced Birefringence in Single Grain Boundaries of PTCR Barium Titanate Ceramics above the Curie Point

Definite electric field-induced birefringence has been observed in the vicinity of single grain boundaries with a distinct positive temperature coefficient of resistivity (PTCR) characteristic in semiconducting barium titanate (BaTiO 3 ) ceramics above the Curie point ( T c = 120°C). Both the highest temperature up to which birefringence could be observed and the temperature giving the maximum resistivity of the PTCR characteristic shifted to higher temperatures with increasing electric field in almost the same manner. Single grain boundaries examined in this study were formed in thin semiconducting BaTiO 3 bars with a diameter of 50-100 w m. The observed birefringence clearly demonstrates that spontaneous polarization ( P s ) induced by electric field can exist in the vicinity of grain boundaries in semiconducting BaTiO 3 ceramics even above T c . Based on the obtained results the mechanism of the PTCR effect may be interpreted by a model that P s plays a decisive role in the determination of the grain-boundary potential barrier height even above T c .

Birefringence study of the freezing mechanism of lanthanum-modified lead zirconate titanate relaxor ferroelectrics

The temperature dependence of the electric field-induced birefringence and electro-optic coefficient of Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 (PLZT) relaxor ferroelectrics were investigated. The remanent birefringence of the poled specimen disappeared around 268 K, thus indicating this to be the freezing temperature. On the other hand, a maximum value of the electro-optic coefficient was observed around 235 K, which is much lower than the freezing temperature. A brief discussion is given on the electro-optic coefficient behavior from the viewpoint of the freezing mechanism of PLZT relaxor ferroelectrics.

Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.

Electric field-induced transient birefringence and light scattering of synthetic liposomes

The dynamics of electric field-induced transient birefringence Δ n( t) and light scattering (detected as turbidity) of 190 nm diameter unilamellar vesicles of dioleoylphosphatidylcholine are investigated as a function of applied field strength E, length of the square pulse Δ t, lipid concentration, mean hydrodynamic diameter 〈 D h〉, ionic strength, and temperature. Generally, induced birefringence exclusively is observed at low lipid concentration and below certain threshold values of E and Δ t, whereas concomitant induced turbidity appears at high lipid concentration and above thresholds values of E and Δ t. Turbidity is monitored through the change in transmitted intensity Δ S ‖( t) and Δ S ⊥( t) of light polarized parallel and perpendicular to the applied field E . The field-induced structural changes are reflected in double-exponential forward relaxation and triple-exponential reverse relaxation of the positive birefringence, and in non-exponential relaxations of Δ S ‖( t) and Δ S ⊥( t). Under the field, the associated physical events are interpreted as elongation of the spherical bilayer shells in the direction of E , linear chain formation (pearling) of the induced dipolar liposomes parallel to E , and partial fusion of adjoining vesicles within the chains. Under conditions where electroporation can be detected, pore opening succeeds the elongation of the vesicles. After termination of the field, the vesicles return to their original time average spherical shape, the oriented chains randomize and disintegrate, and the fused structures are converted either to unilamellar or multilamellar vesicles.

Electric Field Induced Birefringence in Nematic Liquid Crystal Films: Evidence for Wall Quenching of Director Fluctuations

Abstract The effect of an electric field on the measured order parameter of a liquid crystal is considered. It is pointed out that an external field may influence the apparent degree of order by changing the microscopic order parameter, and by quenching director fluctuations. In nematic liquid crystal the latter dominates, but for smectic phases the effects of field-induced molecular order may be observable. Measurements are reported of the electric field-induced birefringence in nematic 44′-n-pentylcyanobiphenyl (5CB) and the nematic and smectic phases of 44′-n-octylcynobiphenyl (8CB). The modular dependence of induced order on electric field strength predeicted for nematics is experimentally confirmed by Fourier analysis of the electro-optic response. At low field strength, and in the smectic phase, the induced order varies as the square of the field strength, and the dependece on cell thickness is not as predicted by continuum theory. By introducing two scaling parameters into the theory it is possible...

Hydrodynamic studies of a DNA-protein complex: Elongation of single stranded nucleic acids upon complexation with the gene 32 protein of phage T4 deduced from electric field-induced birefringence experiments

Short DNA and RNA fragments complexed with the helix destabilizing protein of bacteriophage T4, GP32, have been studied in solution by electric birefringence and circular dichroism. The birefringence of the complexes is positive and the magnitude indicates that the DNA and RNA fragments become linear and rigid upon protein binding. The field free decay is biphasic. On the basis of a rigid rod approximation the slow relaxation time leads to a base-base distance along the helix axis in the complex from 4.3 to 5.6 Å, an elongation of at least 50% compared to single-stranded DNA.

Electric Field-Induced Birefringence in Liquid-Crystalline Blue Phases

The optical behaviour of liquid-crystalline 'Blue Phases' (BP) of polar cholesterogenic compounds in electric Fields up to 100 kV • cm-1 has been investigated by means of polarization microscopy. By an observation parallel to the field direction a birefringence dependent on the Field strength was detected in the BP I. This result is discussed in terms of a non-linear electrooptic effect, showing that the BP I becomes optically biaxial in an electric field. In the BP II a birefringence was not observed under the same conditions probably indicating that the field induces an optical uniaxiality with the optic axis parallel to the field. The results are discussed with respect to possible cubic structures of the BPs