Field-induced Birefringence Research Articles

Generalized Langevin-Debye model of the field dependence of tilt, birefringence, and polarization current near the de Vries smectic-A*to smectic-C*liquid crystal phase transition

In chiral smectic-A (Sm-A) liquid crystals, an applied electric field induces a tilt of the optic axis from the layer normal. When these materials are of the de Vries type, the electroclinic tilt susceptibility is unusually large, with the field-induced director reorientation accompanied by a substantial increase in optical birefringence with essentially no change in the smectic layer spacing. In order to account for the observed electro-optic behavior, we assume that the molecular orientation distribution in the Sm-A has two degrees of freedom: azimuthal orientation and tilt of the molecular long axis from the layer normal, with the tilt confined to a narrow range of angles. We present a generalized Langevin-Debye model of the response of this orientational distribution to applied field that gives a field-induced optic axis tilt, birefringence, and polarization dependence that agrees well with experimental measurements and reproduces the double-peaked polarization current response characteristic of a first-order Sm-A(*)-Sm-C(*) transition. Additionally, we find that the measured field-induced polarization and the Langevin-Debye model predictions can be quantitatively described as pre-transitional behavior near the tricritical point of a recently published generalized 3D XY model of interacting hard rods confined to reorient on a cone in the presence of an applied field.

Hysteresis and memory factor of the Kerr effect in blue phases

The performance of a polymer-stabilized blue phase system based on a nematic host with large dielectric anisotropy and a chiral dopant with high helical twisting power is investigated and the influence of the reactive monomer composition on the electro-optic characteristics is studied. Field-induced birefringence with a Kerr coefficient greater than 1 nm V−2 can be achieved in a large temperature range from well below 20 °C to above 55 °C. The disturbing influences of electro-optic hysteresis and memory effects can be reduced by diligent choice of the composition and appropriate electric addressing.

Effect of dimerization on the field-induced birefringence in ferrofluids

The magnetic-field-induced birefringence in a ferrofluid composed of spherical cobalt nanoparticles has been studied both experimentally and theoretically. The considerable induced birefringence determined experimentally has been attributed to the formation of chains of nanoparticles. The birefringence has been measured as a function of the external magnetic field and the volume fraction (f) of nanoparticles. It is quadratic in f as opposed to the Faraday effect, which is linear in f. Experimental results agree well with the theoretical model based on a simple density functional approach. For dilute solutions the experimental results can be explained by assuming that only dimers of nanoparticles are formed while the concentration of longer chains is negligible.

P.137L: Late‐News Poster: Polymer Dispersed Blue Phase Liquid Crystals

AbstractWe have demonstrated polymer dispersed blue phase (PDBP) liquid crystal films by encapsulating blue phase liquid crystal in a matrix of polymer. The phase‐separated blue phase liquid crystal molecules show droplets with a small size distribution and exhibit a blue phase at room temperature. The PDBP films enable field‐induced birefringence at low switching voltage. Details of preparation of PDBP and their thermal, optical and electro‐optical properties will be presented.

Unusual electro-optical behavior in a wide-temperature BPIII cell

A low driving voltage and fast response blue phase III (BPIII) liquid-crystal device with very low dielectric anisotropy is demonstrated. To stabilize BPIII in a wide temperature range (> 15°C), a chiral molecule with good solubility was chosen. By studying field-dependent polarization state of the transmitting light, it was found that the field-induced birefringence becomes saturated in the high field. However, the transmitting intensity exhibits a tendency to increase as the electric field increases. This indicates that the electro-optical behavior in BPIII device may be from the flexoelectric effect, which induces tilted optical axis and then induces birefringence. Because the phase transition from BPIII to chiral nematic phase does not happen, the device shows no hysteresis effect and no residual birefringence, exhibits fast response, and can be a candidate for fast photonic application.

Aggregation, pretransitional behavior, and optical properties in the isotropic phase of lyotropic chromonic liquid crystals studied in high magnetic fields

We report results on the high-field magneto-optical response of four aqueous, lyotropic, chromonic liquid crystal formulations in the isotropic phase. Measurements of the field-induced birefringence at temperatures above the isotropic-nematic coexistence region at high magnetic fields reveal qualitative differences in different materials; these differences can be attributed to the nature of aggregation and are discussed within the context of competing aggregation models. Extending these measurements to very high fields and large optical phase differences reveals the presence of an unexpected optical phenomenon: magnetic field-induced circular birefringence, measured in the Voigt geometry, in a system containing no molecularly chiral species. Possible origins of this effect are discussed.

Aqueous Suspensions of GdPO4 Nanorods: A Paramagnetic Mineral Liquid Crystal

Colloidal suspensions of rod-like nanoparticles are well-known to readily form liquid-crystalline phases. Using mineral nanoparticles for this purpose may impart their liquid-crystalline suspensions with original physical properties. We synthesized GdPO(4) nanorods whose aqueous suspensions spontaneously organize in a nematic phase at high concentrations. The nematic phase is very well aligned by small magnetic fields, and the isotropic phase displays a very large field-induced birefringence. Moreover, the nanorods migrate to regions of high magnetic field. On the basis of magnetization measurements, we show that this unusual behavior is due to the fact that GdPO(4) nanorods are actually paramagnetic. Such a paramagnetic mineral liquid crystal, easily synthesized and little sensitive to temperature, may be an interesting alternative to organometallic thermotropic liquid crystals for applications where magnetic field alignment would be more suitable than electric field alignment.

Electric-Field-Induced Perfect Anti-Nematic Order in Isotropic Aqueous Suspensions of a Natural Beidellite Clay

We study the electric-field-induced birefringence and orientational order in the isotropic phase of aqueous suspensions of exfoliated natural beidellite clay particles, thin (L = 0.65 nm) flat charged sheets with high aspect ratio, D/L ≈ 300. Our electric birefringence experiment is optimized for aqueous suspensions of colloidal particles, with a high frequency a.c. electric field, ν ≈ 1 MHz, applied by two external electrodes to a thin flat sample, sealed in an optical capillary. In isotropic and biphasic samples, we observed strong field-induced birefringence Δn(E), saturating at moderate E(sat) field to a plateau Δn(sat) proportional to the volume fraction ϕ. The field-induced order parameter S(E) is negative and saturates to S(sat) = -0.5 above E(sat). This corresponds to a perfect "anti-nematic" order, i.e. the normals of the beidellite particles are perpendicular to the field, without any preferred azimuthal direction. The measured specific excess polarizability ΔA(sp) is among the highest data reported for other strongly anisometric dielectric and metal particles. We explain the high ΔA(sp) value with the strong induced polarization of the electric double layer of counterions at the charged particle/electrolyte interface. The estimated equivalent conductivity of the beidellite particle K(eq) = 2 K(σ)/L is several orders of magnitude larger than the bulk conductivity of the electrolyte K(e), resulting in a metal-like behavior of the beidellite disks under field. In the isotropic regions of biphasic nematic/isotropic samples, the excess polarizability is further enhanced by an order of magnitude, indicating collective reorientation of the particles. We propose that this enhancement might be due to pretransitional fluctuations of the spontaneous nematic order S(N) of the colloidal suspension and/or formation of chains of particles, with antinematic order of the beidellite disks in the chains.

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.

Quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped trans-polyisoprene, an organic nanometallic system

Thin films of 1,4-trans-polyisoprene have been prepared on various substrates from toluene solution and characterized using Fourier transform infrared (FTIR) spectroscopy, optical absorption spectroscopy and X-ray diffraction before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at 4.2 eV and the other at 3.2 eV. X-ray diffraction indicates an increase of (111) and (122) peak intensities upon doping. Quadratic electro-optic measurements have been made using field-induced birefringence. The Kerr coefficients as measured (3.5×10 −10 m/V 2 at 633 nm and 2.5×10 −10 m/V 2 at 1.55 μm) are exceptionally large, and they have been attributed to the subnanometer-size metallic domains formed upon doping and charge transfer.

Relativistic birefringence induced by a high-intensity laser field in a plasma

Field-induced birefringence, also known as cross-polarization wave generation, has played an important role in ultrafast nonlinear optics. In this paper we analyze birefringence induced by relativistic collective motion of electrons driven by a high-intensity laser field. An analytical expression for the phase difference between the parallel and perpendicular polarizations of a weak probe pulse with respect to the polarization of a strong pump pulse as a function of intensity, density, and wavelengths is derived. It is shown that under typical experimental conditions of high-field physics, the effect is well above detection threshold. The analysis is compared with particle-in-cell simulations, and the agreement provides good support for the theory.

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.

Quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped poly( [formula omitted]-pinene) measured at longer wavelengths including [formula omitted

The quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped poly( β -pinene), measured at longer wavelengths including 1.55 μm, is reported. The field-induced birefringence technique was used. A modulation depth of 0.1% was observed in a 1 μm thick sample for an applied field of 1 V/μm at 1.55 μm. The corresponding Kerr coefficient obtained is 1.6×10 −10 m/V 2. The Kerr coefficients measured at 790–810 nm are also very large. This exceptionally large (largest known) optical nonlinearity has been attributed to the subnanometer size metallic quantum dots formed in this polymer upon doping with iodine and the optimized film quality recently achieved.

Evolution of Subphases in a Prototype Binary Mixture System as Observed by Electric-Field-Induced Birefringence and Helical Pitch

We have constructed the E–T phase diagrams by drawing the field-induced birefringence contours in the prototype MHPOCBC and MHPOOCBC binary mixture system. The obtained subphase evolution in the diagrams clearly indicate that there exist some additional biaxial subphases other than the ordinary ones with three- and four-layer superstructures, and that the field-induced deformation of uniaxial in MHPOCBC occurs quite differently in the high and low temperature regions. We have discussed the origin and structures of the additional subphases and different types of the field-induced deformations in terms of the extended Emelyanenko-Osipov model together with the Devil's staircase character of field-induced unwinding process of the short-pitch helical structure published by Yamashita et al. in these Proceedings.

Single-crystal film of a combination of organic materials involving a molecular salt and a laser dye; Electro-optic effect

Organic single-crystal films involving a combination of a molecular salt (DAST) and a laser dye (IR-125) have been grown on quartz substrates using the modified shear method. Polarized optical microscopy, polarization-dependent optical absorption, x-ray diffraction and electro-optic effect have been measured to characterize the film. The dipole axis of DAST is oriented close to parallel to the substrate. The IR-125 molecule appears to be oriented close to perpendicular to the dipole axis of DAST. The optical gap of the film is about 1.4 eV. The electro-optic coefficient ( r 11 ) as measured by the field-induced birefringence is ∼300 pm/V at 633 nm. Such films are promising for various photonic applications.

Degeneracy lifting near the frustration points due to long-range interlayer interaction forces and the resulting varieties of polar chiral tilted smectic phases

To gain a clear understanding of ferroelectricity and antiferroelectricity together with their frustration in chiral tilted smectic liquid crystals, we have constructed E–T phase diagrams by drawing field-induced birefringence contours in the prototype binary mixture system of MHPOCBC and MHPOOCBC. The results obtained are discussed in terms of the theoretical model proposed by Emelyanenko and Osipov; we have insisted on the appropriateness of specifying the biaxial subphases, which emerge sequentially in the temperature-induced transition, by the relative ratio of ferroelectric and antiferroelectric orderings in the superstructure unit cell, such as , where qT = [F]/([A] + [F]). Additional subphases other than the ordinary subphases with three- and four-layer superstructures, and , have been established to exist, firmly for and less adequately for and . Likewise, we have observed several stable superstructures during the field-induced transition from the biaxial subphases to unwound SmC*, and have tried to specify them using qE = |[R] − [L]|/([R] + [L]), where [R] and [L] refer to the numbers of smectic layers with directors tilted to the right and to the left, respectively, in a unit cell of the superstructure. We have also found the characteristic field-induced deformation of the uniaxial subphase and attempted to understand it in terms of the devil's staircase due to soliton condensation reported recently by Torikai and Yamashita (41).

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.

Optical retardation of in-plane switched polymer dispersed liquid crystals

We investigated the field-induced optical birefringence of a photopolymerized polymer dispersed liquid crystal (PDLC) with nearly a 50% volume ratio of phase-separated liquid crystal (LC) droplets. Measurements were performed in the in-plane switching configuration, which was attained by the use of interdigitated electrodes. The observed dependence of the sample transmittance on the external field magnitude is independent of the polarization state of the incident beam. The material transforms a linearly polarized wave into an elliptically polarized wave. Phase retardation between the two eigenpolarizations is proportional to the order parameter of the LC domains. The observed phenomena are attributed to the dominant role of the droplet proximity effects in the optical properties of the material. Dynamic response of the retardation takes place in a broad range of time scales, which are associated with relaxation of different degrees of freedom of the system. The observed dynamic features indicate an interconnected structure of the LC domains in the LC rich PDLC structures.